IPv6 addressing will eventually replace IPv4 addressing although both types of addresses will coexist for the foreseeable future. IPv6 overcomes the limitations of IPv4 and has features that better suit current and foreseeable network demands. The 32-bit IPv4 address space provides approximately 4,294,967,296 unique addresses.
IPv6 address space provides 340,282,366,920,938,463,463,374,607,431,768,211,456 addresses, or 340 undecillion addresses, which is roughly equivalent to the number of grains of sand on Earth. Table 10-1 provides a visual to compare the IPv4 and IPv6 address space.
Table 10-1 Number of Zeros for Increasing Levels of Scientific Notation
Number Name
Scientific Notation
Number of Zeros
1 Thousand
10 3
1,000
1 Million
10 6
1,000,000
1 Billion
10 9
1,000,000,000
1 Trillion
10 12
1,000,000,000,000
1 Quadrillion
10 15
1,000,000,000,000,000
1 Quintillion
10 18
1,000,000,000,000,000,000
1 Sextillion
10 21
1,000,000,000,000,000,000,000
1 Septillion
10 24
1,000,000,000,000,000,000,000,000
1 Octillion
10 27
1,000,000,000,000,000,000,000,000,000
1 Nonillion
10 30
1,000,000,000,000,000,000,000,000,000,000
1 Decillion
10 33
1,000,000,000,000,000,000,000,000,000,000,000
1 Undecillion
10 36
1,000,000,000,000,000,000,000,000,000,000,000,000
The following are other benefits of the IPv6 protocol:
There is no need for NAT. Each device can have its own globally routable address.
The designers of IPv6 thought that it would be adopted quickly, as the number of remaining available IPv4 address blocks was decreasing rapidly. Initial estimates were that IPv6 would be globally deployed by 2003. Obviously, these estimates were incorrect.
There is no specific date to move to IPv6. Both IPv4 and IPv6 will coexist in the near future, and the transition is taking several years. The IETF has created various protocols and tools to help network administrators migrate their networks to IPv6. The migration techniques can be divided into three categories: dual stack, tunneling, and translation.
Dual Stack
Dual stack enables IPv4 and IPv6 to coexist on the same network segment, as shown in Figure 10-6. Dual stack devices run both IPv4 and IPv6 protocol stacks simultaneously. Known as native IPv6, this means the customer network has an IPv6 connection to its ISP and is able to access content found on the Internet over IPv6.
Figure 10-6 A Dual Stack Topology
Tunneling
Tunneling is a method of transporting an IPv6 packet over an IPv4 network, as shown in Figure 10-7. The IPv6 packet is encapsulated inside an IPv4 packet, similar to other types of data.
Figure 10-7 Routing IPv6 Packets Inside an IPv4 Tunnel
Translation
Network Address Translation 64 (NAT64) enables IPv6-enabled devices to communicate with IPv4-enabled devices using a translation technique similar to NAT for IPv4. An IPv6 packet is translated to an IPv4 packet, and an IPv4 packet is translated to an IPv6 packet. The NAT64 router translates the different IP addresses between networks (the solid line) so that the PCs with different IP addresses can communicate (the dotted line), as shown in Figure 10-8.
IPv6 is more than just larger address space. A new IP protocol was an opportunity to make performance improvements and provide much-needed new features.
In addition to the increase in length, IPv6 addresses have other characteristics that are different than IPv4 addresses. Among the differences are the following:
Address autoconfiguration—Stateless Address Autoconfiguration (SLAAC) allows a host to create its own Internet-routable address (global unicast address, or GUA) without the need for a DHCP server. As shown in Figure 10-9, with the default method, the host receives the prefix (network address), prefix length (subnet mask), and default gateway from the Router Advertisement message of the router. The host can then create its own unique interface ID (host portion of the address) to give itself a routable global unicast address.
Figure 10-9 SLAAC Operation
Link-local address—A link-local address is used when communicating with a device on the same network.
The developers of IPv6 made improvements to IP and related protocols such as ICMPv6. These improvements include features related to efficiency, scalability, mobility, and flexibility for future enhancements.
A router is a Layer 3 intermediary device that performs the packet forwarding or routing. Routers have routing tables that contain the information the router needs to forward the packet.
How does the router determine which interface to use to send the message on a path to get to the destination network? Each port, or interface, on a router connects to a different local network. Every router contains a table of all locally connected networks and the interfaces that connect to them. These routing tables can also contain information about the routes, or paths, that the router uses to reach other remote networks that are not locally attached.
When a router receives a frame, it de-encapsulates the frame to get to the packet containing the destination IP address. It matches the network portion of the destination IP address to the networks that are listed in the routing table. If the destination network address is in the table, the router encapsulates the packet into a new frame to send it out. (Note that it also inserts a new destination MAC address and recalculates the FCS field in the new frame.) It forwards the new frame out of the interface associated with the path to the destination network, as shown in Figure 7-6. The process of forwarding the packets toward their destination network is called routing.
Figure 7-6 A Router Selecting the Path to the Destination
Router interfaces do not forward messages that are addressed to the local network broadcast IP address. As a result, local network broadcasts are not sent across routers to other local networks.
Video—Messages Within and Between Networks—Part 1 (7.2.3)
Refer to the online course to view this video.
Video—Messages Within and Between Networks—Part 2 (7.2.4)
A router forwards a packet to one of two places: a directly connected network containing the destination host or to another router on the path to reach the destination host. When a router encapsulates the frame to forward it out an Ethernet interface, it must include a destination MAC address. This is the MAC address of the destination host, if the destination host is part of a network that is locally connected to the router. Figure 7-7 shows a host sending a packet to a host on the same network.
Figure 7-7 Destination Host Is on the Same Local Network
If the router must forward the packet to another router through an Ethernet interface, it uses the MAC address of the connected router. Routers obtain these MAC addresses from ARP tables.
Each router interface is part of the local network to which it is attached and maintains its own ARP table for that network, as shown in Figure 7-8. The ARP tables contain the MAC addresses and IPv4 addresses of all the individual hosts on that network.
Figure 7-8 Destination Host Is on a Remote Network
Routers in the distribution layer can be used to interconnect local networks at various locations of an organization that are geographically separated, as shown in Figure 7-3.
Routers in the distribution layer can be used to logically group users, such as departments within a company, who have common needs or for access to resources, as shown in Figure 7-4.
Figure 7-4 Logically Separating Users into Groups
When Routing Is Needed (7.1.3)
In most situations, you want your devices to be able to connect beyond your local network: out to other homes, businesses, and the Internet. Devices that are beyond the local network segment are known as remote hosts. When a source device sends a packet to a remote destination device, the help of routers and routing is needed. Routing is the process of identifying the best path to a destination.
A router is a networking device that connects multiple Layer 3 IP networks. At the distribution layer of the network, routers direct traffic and perform other functions critical to efficient network operation. Routers, like switches, are able to decode and read the messages that are sent to them. Unlike switches, which make their forwarding decision based on the Layer 2 MAC address, routers make their forwarding decision based on the Layer 3 IP address, as shown in Figure 7-5.
Figure 7-5 IP Packet Encapsulated in an Ethernet Frame
The packet format contains the IP addresses of the destination and source hosts, as well as the message data being sent between them. The router reads the network portion of the destination IP address and uses it to find which one of the attached networks is the best way to forward the message to the destination.
Any time the network portion of the IP addresses of the source and destination hosts do not match, a router must be used to forward the message. If a host located on network 1.1.1.0 needs to send a message to a host on network 5.5.5.0, the host forwards the message to the router. The router receives the message, de-encapsulates the Ethernet frame, and then reads the destination IP address in the IP packet. It then determines where to forward the message. It re-encapsulates the packet back into a new frame and forwards the frame on to its destination.
Lab—IPv4 Addresses and Network Communication (7.1.4)
In this lab, you will complete the following objectives:
Build a simple peer-to-peer network and verify physical connectivity.
Assign various IP addresses to hosts and observe the effects on network communication.
Creating your own peer-to-peer network is fun, but pretty soon you‛ll want to venture out to other networks and onto the Internet. When that time comes, you will need to have a router. This part of networking is even more fun.
Most network communication involves sending packets over multiple networks. Routing is the process of forwarding IP packets from one network to another network.
As networks grow, it is often necessary to divide one access layer network into multiple access layer networks. There are many ways to divide networks based on different criteria:
Broadcast containment
Security requirements
Physical locations
Logical grouping
The distribution layer connects these independent local networks and controls the traffic flowing between them. It is responsible for ensuring that traffic between hosts on the local network stays local. Only traffic that is destined for other networks is passed on. The distribution layer can also filter incoming and outgoing traffic for security and traffic management.
Networking devices that make up the distribution layer are designed to interconnect networks, not individual hosts. Individual hosts are connected to the network via access layer devices, such as switches. The access layer devices are connected to each other via distribution layer devices, such as routers.
Routers in the distribution layer can limit broadcasts to the local network where they need to be heard (see Figure 7-1). Although broadcasts are necessary, too many hosts connected on the same local network can generate excessive broadcast traffic and slow down the network.
Figure 7-1 Broadcast Containment
Security Requirements
Routers in the distribution layer can separate and protect certain groups of computers where confidential information resides, as shown in Figure 7-2. Routers can also hide the addresses of internal computers from the outside world to help prevent attacks and control who can get into or out of the local network.
1. Which destination address is used in an ARP request frame?
AAAA.AAAA.AAAA
255.255.255.255
The physical address of the destination host
0.0.0.0
FFFF.FFFF.FFFF
2. Which network device can serve as a boundary to divide a Layer 2 broadcast domain?
Access point
Ethernet hub
Ethernet bridge
Router
3. Which term refers to the process of placing one message format inside another message format?
Encapsulation
Manipulation
Encoding
Segmentation
4. What is the purpose of the core layer in the Cisco hierarchical network design model?
High-speed backbone switching
Aggregation point for smaller networks
Network access to end devices
Flow control between networks
5. Which network device has the primary function to send data to a specific destination based on the information found in the MAC address table?
Modem
Switch
Router
Hub
6. Refer to the exhibit. How is a frame sent from PCA forwarded to PCC if the MAC address table on switch SW1 is empty?
SW1 floods the frame on all ports on the switch, excluding the interconnected port to switch SW2 and the port through which the frame entered the switch.
SW2 drops the frame because it does not know the destination MAC address.
SW1 floods the frame directly to SW2. SW2 floods the frame to all ports connected to SW2, excluding the port through which the frame entered the switch.
SW1 floods the frame on all ports on SW1, excluding the port through which the frame entered the switch.
7. What information does an Ethernet switch examine and use to build its address table?
Destination MAC address
Destination IP address
Source MAC address
Source IP address
8. Which fields are found in an 802.3 Ethernet frame? (Choose three.)
Source logical address
Source physical address
Destination physical address
Frame check sequence
Destination logical address
Media type identifier
9. Which devices would commonly be found at the access layer of the hierarchical enterprise LAN design model? (Choose two.)
Layer 2 switch
Access point
Layer 3 device
Firewall
Modular switch
10. Which statement is true about broadcast and collision domains?
Adding a switch to a network will increase the size of the broadcast domain.
The size of the collision domain can be reduced by adding hubs to a network.
Adding a router to a network will increase the size of the collision domain.
The more interfaces a router has, the larger the resulting broadcast domain.
11. How much data can be encapsulated into a normal-sized Ethernet frame before it is sent over the network?
23 to 1500 bytes
46 to 1500 bytes
64 to 1518 bytes
0 to 1024 bytes
12. What is the purpose of ARP in an IPv4 network?
To forward data onward based on the destination MAC address
To forward data onward based on the destination IPv4 address
To obtain a specific MAC address when an IPv4 address is known
To build the MAC address table in a switch from the information that is gathered
At times, an end device may need to send an Ethernet frame to all devices on the same Ethernet LAN. Although these Ethernet broadcasts are common, it is important that they are kept to a minimum so they do not affect the overall performance of the network.
Within the local network, one host often needs to be able to send messages to all the other hosts at the same time. This can be done using a message known as a broadcast. Broadcasts are useful when a host needs to find information without knowing exactly what other hosts can supply it, or when a host wants to provide information to all other hosts in the same network in a timely manner.
A message can contain only one destination MAC address. So, how is it possible for a host to contact every other host on the local network without sending out a separate message to each individual MAC?
To solve this problem, broadcast messages are sent to a unique MAC address that is recognized by all hosts. The broadcast MAC address is actually a 48-bit address made up of all ones. Because of their length, MAC addresses are usually represented in hexadecimal notation. The broadcast MAC address in hexadecimal notation is FFFF.FFFF.FFFF. Each F in the hexadecimal notation represents four ones in the binary address.
Figure 6-16 shows H1 sending out a broadcast message on a LAN. All the other devices receive the broadcast.
When a host receives a message addressed to the broadcast address, it accepts and processes the message as though the message was addressed directly to it. When a host sends a broadcast message, switches forward the message to every connected host within the same local network. For this reason, a local-area network, a network with one or more Ethernet switches, is also referred to as a broadcast domain.
If too many hosts are connected to the same broadcast domain, broadcast traffic can become excessive. The number of hosts and the amount of network traffic that can be supported on the local network are limited by the capabilities of the switches used to connect them. As the network grows and more hosts are added, network traffic, including broadcast traffic, increases. To improve performance, often one local network must be divided into multiple networks, or broadcast domains, as shown in Figure 6-17. Routers are used to divide the network into multiple broadcast domains.
Figure 6-17 Broadcast Domains Segmented by a Router
On a local Ethernet network, a NIC accepts a frame only if the destination address is the broadcast MAC address or else corresponds to the MAC address of the NIC.
Most network applications, however, rely on the logical destination IP address to identify the location of the servers and clients. Figure 6-18 illustrates the problem that arises if a sending host only has the logical IP address of the destination host. How does the sending host determine what destination MAC address to place within the frame?
Figure 6-18 A Host Needs the IPv4 Address of the Destination
The sending host can use an IPv4 protocol called Address Resolution Protocol (ARP) to discover the MAC address of any host on the same local network. IPv6 uses a similar method known as Neighbor Discovery.
ARP (Address Resolution Protocol) is a process used by devices when they know the IPv4 address of a device but do not know that device‛s Ethernet MAC address. ARP uses a three-step process to discover and store the MAC address of a host on the local network when only the IPv4 address of the host is known:
The sending host creates and sends a frame addressed to a broadcast MAC address. Contained in the frame is a message with the IPv4 address of the intended destination host.
Each host on the network receives the broadcast frame and compares the IPv4 address inside the message with its configured IPv4 address. The host with the matching IPv4 address sends its MAC address back to the original sending host.
The sending host receives the message and stores the MAC address and IPv4 address information in a table called an ARP table.
When the sending host has the MAC address of the destination host in its ARP table, it can send frames directly to the destination without doing an ARP request. Because ARP messages rely on broadcast frames to deliver the requests, all hosts in the local IPv4 network must be in the same broadcast domain (see Figure 6-19).
Figure 6-19 A Host Using ARP to Determine the IPv4 Address
Lab—View Captured Traffic in Wireshark (6.4.8)
In this lab, you will complete the following objectives:
The following is a summary of each topic in the chapter:
Encapsulation and the Ethernet Frame—The process of placing one message format (such as a letter) inside another message format (such as an envelope) is called encapsulation. Each computer message is encapsulated in a specific format, called a frame, before it is sent over the network. A frame acts like an envelope; it provides the address of the intended destination and the address of the source host. The format and contents of a frame are determined by the type of message being sent and the channel over which it is communicated.
The Ethernet protocol standards define many aspects of network communication including frame format, frame size, timing, and encoding. When messages are sent between hosts on an Ethernet network, the hosts format the messages into the frame layout that is specified by the standards. Frames are also referred to as Layer 2 PDUs. The reason is that the protocols that provide the rules for the creation and format of the frame perform the functions that are specified at the data link layer of the OSI model.
Hierarchical Network Design—IP addresses contain two parts. One part identifies the local network. This portion of the IP address is the same for all hosts connected to the same local network. The second part of the IP address identifies the individual host. Both the physical MAC and logical IP addresses are required for a computer to communicate on a hierarchical network, just like both the name and address of a person are required to send a letter. Large Ethernet networks consisting of many hosts need to be divided into smaller, more manageable pieces. One way to divide larger networks is to use a hierarchical design model. The hierarchical design has three basic layers:
Access Layer—This layer provides connections to hosts in a local Ethernet network.
Distribution Layer—This layer interconnects the smaller local networks.
Core Layer—This layer provides a high-speed connection between distribution layer devices.
With a hierarchical design, you need a logical addressing scheme that can identify the location of a host. The most common addressing scheme used on company networks is Internet Protocol version 4 (IPv4). Internet Protocol version 6 (IPv6) is the network layer protocol currently being implemented as a replacement to IPv4.
The Access Layer—The access layer is the part of the network in which people gain access to other hosts and to shared files and printers. The access layer provides the first line of networking devices that connect hosts to the wired Ethernet network. Several types of networking devices can be used to connect hosts at the access layer, including Ethernet hubs and switches.
Ethernet hubs contain multiple ports that are used to connect hosts to the network. Hubs cannot decode the messages sent between hosts on the network. Hubs cannot determine which host should get any particular message. A hub simply accepts electronic signals from one port and regenerates (or repeats) the same message out all of the other ports. All hosts attached to the hub share the bandwidth and receive the message. Hosts ignore the messages that are not addressed to them. Only the host specified in the destination address of the message processes the message and responds to the sender.
If a switch is being used and the destination MAC address is not in the MAC address table, the switch cannot determine where the destination host is located. The switch then uses a process called flooding to forward the message out to all attached hosts except for the sending host. How does the MAC address of a new host get into the MAC address table? A switch builds the MAC address table by examining the source MAC address of each frame that is sent between hosts. When a new host sends a message or responds to a flooded message, the switch immediately learns its MAC address and the port to which it is connected. The table is dynamically updated each time a new source MAC address is read by the switch.
Broadcast Containment—Within the local network, a host may need to send messages to all the other hosts at the same time. This can be done using a broadcast message. Broadcast messages are sent to a unique MAC address that is recognized by all hosts. The broadcast MAC address is actually a 48-bit address made up of all ones.
When a host receives a message addressed to the broadcast address, it accepts and processes the message as though the message was addressed directly to it. When a host sends a broadcast message, switches forward the message to every connected host within the same local network. For this reason, a LAN is also referred to as a broadcast domain. Routers are used to divide the network into multiple broadcast domains.
How does the sending host determine what destination MAC address to place within the frame? The sending host can use an IPv4 protocol called ARP to discover the MAC address of any host on the same local network. IPv6 uses a similar method known as Neighbor Discovery. ARP uses a three-step process to discover and store the MAC address of a host on the local network when only the IPv4 address of the host is known:
The sending host creates and sends a frame addressed to a broadcast MAC address. Contained in the frame is a message with the IPv4 address of the intended destination host.
Each host on the network receives the broadcast frame and compares the IPv4 address inside the message with its configured IPv4 address. The host with the matching IPv4 address sends its MAC address back to the original sending host.
The sending host receives the message and stores the MAC address and IPv4 address information in a table called an ARP table.
An Ethernet switch is a device that is used at the access layer. When a host sends a message to another host connected to the same switched network, the switch accepts and decodes the frames to read the physical (MAC) address portion of the message, and then sends the message to the destination, as shown in Figure 6-11.
Figure 6-11 A Switch Operation
A table on the switch, called a MAC address table, contains a list of all of the active ports and the host MAC addresses that are attached to them. When a message is sent between hosts, the switch checks to see if the destination MAC address is in the table. If it is, the switch builds a temporary connection, called a circuit, between the source and destination ports. This new circuit provides a dedicated channel over which the two hosts can communicate. Other hosts attached to the switch do not share bandwidth on this channel and do not receive messages that are not addressed to them. A new circuit is built for every new conversation between hosts. These separate circuits allow many conversations to take place at the same time, without collisions occurring. Ethernet switches also allow for the sending and receiving of frames over the same Ethernet cable simultaneously. This improves the performance of the network by eliminating collisions.
What happens when the switch receives a frame addressed to a new host that is not yet in the MAC address table? If the destination MAC address is not in the table, the switch does not have the necessary information to create an individual circuit. When the switch cannot determine where the destination host is located, it uses a process called flooding to forward the message out to all attached hosts except for the sending host. Each host compares the destination MAC address in the message to its own MAC address, but only the host with the correct destination address processes the message and responds to the sender.
How does the MAC address of a new host get into the MAC address table? A switch builds the MAC address table by examining the source MAC address of each frame that is sent between hosts. When a new host sends a message or responds to a flooded message, the switch immediately learns its MAC address and the port to which it is connected. The table is dynamically updated each time the switch reads a new source MAC address. In this way, a switch quickly learns the MAC addresses of all attached hosts. Figures 6-12 through 6-15 demonstrate this operation.
Figure 6-12 Source Sends a Message to the Destination
In Figure 6-12, Source PC H3 sends data to Destination PC H7. The switch does not yet have a MAC address for H7.
In Figure 6-13, the switch floods the frame received from H3 out every other port.
Figure 6-13 Switch Floods the Message
In Figure 6-14, after H7 receives the frame, the IP address of the encapsulated packet matches H7‛s IP address. Therefore, H7 replies to H3.
Figure 6-14 The Destination Replies to the Message
In Figure 6-15, the switch updates its table with the MAC address for H7 to map the MAC address to the port.
Figure 6-15 The Switch Records the MAC Address for the Destination
The distribution layer provides a connection point for separate networks and controls the flow of information between the networks. It typically contains more powerful switches, such as the Cisco C9300 series shown in Figure 6-7, than the access layer as well as routers for routing between networks. Distribution layer devices control the type and amount of traffic that flows from the access layer to the core layer.
Figure 6-7 Cisco C9300 Series
Core Layer
The core layer is a high-speed backbone layer with redundant (backup) connections. It is responsible for transporting large amounts within the network. Core layer devices typically include very powerful, high-speed switches and routers, such as the Cisco Catalyst 9600 shown in Figure 6-8. The main goal of the core layer is to transport data quickly.
The access layer describes the network components used to provide an end device access to the network and the LAN.
Access Layer Devices (6.3.1)
The access layer is the basic level of the network. It is the part of the network in which people gain access to other hosts and to shared files and printers. The access layer provides the first line of networking devices that connect hosts to the wired Ethernet network.
Networking devices enable you to connect many hosts with each other and also provide those hosts access to services offered over the network. Unlike the simple network consisting of two hosts connected by a single cable, in the access layer, each host is connected to a networking device. This type of connectivity is shown in Figure 6-9.
Figure 6-9 Multiple Hosts Connected to a Networking Device
Within an Ethernet network, each host is able to connect directly to an access layer networking device using an Ethernet cable. These cables are manufactured to meet specific Ethernet standards. Each cable is plugged into a host NIC and then into a port on the networking device. Several types of networking devices can be used to connect hosts at the access layer, including Ethernet switches.
The original Ethernet networks connected all hosts with a single cable, similar to how TV cables are connected in your home. All users on the network shared the bandwidth available on the cable. As Ethernet networks became more popular, connecting everyone on a single cable was no longer practical nor even possible. Engineers developed a different type of network technology that made it easier to connect and reconnect multiple devices to the network. The first of these types of networking devices were Ethernet hubs.
Hubs contain multiple ports that are used to connect hosts to the network. Hubs are simple devices that do not have the necessary electronics to decode the messages sent between hosts on the network. They cannot determine which host should get any particular message. A hub simply accepts electronic signals from one port and regenerates (or repeats) the same message out all of the other ports. All hosts attached to the hub share the bandwidth and receive the message. Hosts ignore the messages that are not addressed to them. Only the host specified in the destination address of the message processes the message and responds to the sender.
Only one message can be sent through an Ethernet hub at a time. It is possible for two or more hosts connected to a hub to attempt to send a message at the same time. If this happens, the electronic signals that make up the messages collide with each other at the hub. This is known as a collision. The message is unreadable by hosts and must be retransmitted. The area of the network where a host can receive a garbled message resulting from a collision is known as a collision domain.
Because excessive retransmissions can clog up the network and slow down network traffic, hubs are now considered obsolete and have been replaced by Ethernet switches.
Imagine how difficult communication would be if the only way to send a message to someone was to use the person‛s name. If there were no street addresses, cities, towns, or country boundaries, delivering a message to a specific person across the world would be nearly impossible.
On an Ethernet network, the host MAC address is similar to a person‛s name. A MAC address indicates the individual identity of a specific host, but it does not indicate where on the network the host is located. If all hosts on the Internet (millions and millions of them) were each identified by their unique MAC address only, imagine how difficult it would be to locate a single one.
Additionally, Ethernet technology generates a large amount of broadcast traffic so that hosts are able to communicate. Broadcasts are sent to all hosts within a single network. Broadcasts consume bandwidth and slow network performance. What would happen if the millions of hosts attached to the Internet were all in one Ethernet network and were using broadcasts?
For these two reasons, large Ethernet networks consisting of many hosts are not efficient. It is better to divide larger networks into smaller, more manageable pieces. One way to divide larger networks is to use a hierarchical design model.
Video—Benefits of a Hierarchical Network Design (6.2.6)
In networking, hierarchical design is used to group devices into multiple networks that are organized in a layered approach. This method of designing networks consists of smaller, more manageable groups that allow local traffic to remain local. Only traffic that is destined for other networks is moved to a higher layer.
A hierarchical, layered design provides increased efficiency, optimization of function, and increased speed. It allows the network to scale as required because additional local networks can be added without impacting the performance of the existing ones.
As shown in Figure 6-5, the hierarchical design has three basic layers:
Figure 6-5 A Three-Layer Hierarchical Design
Access Layer—This layer provides connections to hosts in a local Ethernet network.
Distribution Layer—This layer interconnects the smaller local networks.
Core Layer—This layer provides a high-speed connection between distribution layer devices.
With a hierarchical design, there is a need for a logical addressing scheme that can identify the location of a host. The most common addressing scheme on the Internet is IPv4. IPv6 is the network layer protocol currently being implemented as a replacement for IPv4. IPv4 and IPv6 will coexist for the foreseeable future. From this point on in this course, the term IP refers to both IPv4 and IPv6.
Access, Distribution, and Core (6.2.8)
IP traffic is managed based on the characteristics and devices associated with each of the three layers of the hierarchical network design model: access, distribution, and core.
The access layer provides a connection point for end-user devices to the network and allows multiple hosts to connect to other hosts through a network device, usually a switch, such as the Cisco 2960-XR shown in Figure 6-6, or a wireless access point. Typically, all devices within a single access layer have the same network portion of the IP address.
Figure 6-6 Cisco 2960-XR
If a message is destined for a local host, based on the network portion of the IP address, the message remains local. If it is destined for a different network, it is passed up to the distribution layer. Switches provide the connection to the distribution layer devices, usually a Layer 3 device such as a router or Layer 3 switch.
The Ethernet protocol standards define many aspects of network communication including frame format, frame size, timing, and encoding.
When messages are sent between hosts on an Ethernet network, the hosts format the messages into the frame layout that is specified by the standards. Frames are also referred to as Layer 2 protocol data units (PDUs). The reason is that the protocols that provide the rules for the creation and format of the frame perform the functions that are specified at the data link layer (Layer 2) of the OSI model.
The format, shown in Figure 6-3, for Ethernet frames, specifies the location of the destination and source MAC addresses and additional information including
Figure 6-3 Ethernet Frame Structure and Field Size
Preamble for sequencing and timing
Start of frame delimiter
Length and type of frame
Frame check sequence to detect transmission errors
The size of Ethernet frames is normally limited to a maximum of 1518 bytes and a minimum size of 64 bytes from the Destination MAC Address field through the frame check sequence (FCS). The preamble and the start of frame delimiter (SFD) are used to indicate the beginning of the frame. They are not used in the calculation of the frame size. Frames that do not match these limits are not processed by the receiving hosts. In addition to the frame formats, sizes, and timing, Ethernet standards define how the bits making up the frames are encoded onto the channel. Bits are transmitted as either electrical impulses over copper cable or as light impulses over fiber-optic cable.
The two different types of addresses are logical addresses and physical addresses. Both of these types have a specific function in ensuring a message can be sent between two devices on the same network or between two devices on different networks.
A person‛s name usually does not change. A person‛s address, on the other hand, relates to where the person lives and can change. On a host, the MAC address does not change; it is physically assigned to the host NIC and is known as the physical address. The physical address remains the same regardless of where the host is placed on the network.
The IP address is similar to a person‛s address. It is known as a logical address because it is assigned logically based on where the host is located. The IP address, or network address, is assigned to each host by a network administrator based on the local network.
IP addresses contain two parts. One part identifies the network portion. The network portion of the IP address is the same for all hosts connected to the same local network. The second part of the IP address identifies the individual host on that network. Within the same local network, the host portion of the IP address is unique to each host, as shown in Figure 6-4.
Figure 6-4 Network and Host Portion of the IPv4 Address
Both the physical MAC and logical IP addresses are required for a computer to communicate on a hierarchical network, just like both the name and address of a person are required to send a letter.
Video—View Network Information on My Device (6.2.3)
Refer to the online course to view this video.
Lab—View Wireless and Wired NIC Information (6.2.4)
In this lab, you will complete the following objectives:
