Category Archives: Cisco The Internet Protocol

Broadcast Transmission (8.6.4)–Cisco The Internet Protocol

Broadcast packets are sent to all hosts in the network using a broadcast address. With a broadcast, the packet contains a destination IPv4 address with all ones (1s) in the host portion. This means that all hosts on that local network (broadcast domain) will receive and look at the packet. Many network protocols, such as DHCP, use broadcasts. When a host receives a packet sent to the network broadcast address, the host processes the packet as it would a packet addressed to its unicast address.

Broadcast may be directed or limited. A directed broadcast is sent to all hosts on a specific network. For example, in Figure 8-11 a host on the 172.16.4.0/24 network sends a packet to 172.16.4.255. A limited broadcast is sent to 255.255.255.255. By default, routers do not forward broadcasts.

Figure 8-11 Broadcast Transmission

When a packet is broadcast, it uses resources on the network and causes every receiving host on the network to process the packet. Therefore, broadcast traffic should be limited so that it does not adversely affect the performance of the network or devices. Because routers separate broadcast domains, subdividing networks can improve network performance by eliminating excessive broadcast traffic.

Video—IPv4 Multicast (8.6.5)

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Multicast Transmission (8.6.6)

Multicast transmission reduces traffic by allowing a host to send a single packet to a selected set of hosts that subscribe to a multicast group.

IPv4 has reserved the 224.0.0.0 to 239.255.255.255 addresses as a multicast range. The IPv4 multicast addresses 224.0.0.0 to 224.0.0.255 are reserved for multicasting on the local network only. These addresses are to be used for multicast groups on a local network. A router connected to the local network recognizes that these packets are addressed to a local network multicast group and never forwards them further. A typical use of a reserved local network multicast address is in routing protocols using multicast transmission to exchange routing information. For example, 224.0.0.9 is the multicast address used by Routing Information Protocol (RIP) version 2 to communicate with other RIPv2 routers.

Hosts that receive particular multicast data are called multicast clients. The multicast clients use services requested by a client program to subscribe to the multicast group.

Each multicast group is represented by a single IPv4 multicast destination address, as shown in Figure 8-12. When an IPv4 host subscribes to a multicast group, the host processes packets addressed to this multicast address, and packets addressed to its uniquely allocated unicast address.

Figure 8-12 Multicast Transmission 

Activity—Unicast, Broadcast, or Multicast (8.6.7)

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Summary (8.7)

The following is a summary of each topic in the chapter:

• Purpose of an IPv4 Address—The IPv4 address is a logical network address that identifies a particular host. An IPv4 address is assigned to the network interface connection for a host. This connection is usually a NIC installed in the device. Every packet sent across the Internet has a source and destination IPv4 address.
• Binary Conversion of an IPv4 Address—An IPv4 address is a series of 32 binary bits (ones and zeros). The 32 bits are grouped into four 8-bit bytes called octets. Each octet is presented as its decimal value, separated by a decimal point or period, called dotted-decimal notation. Each octet is made up of 8 bits, and each bit has a value. The value of each of the four octets can range from 0 to a maximum of 255. Determine the value of the octet by adding the values of positions wherever there is a binary 1 present:
  • If there is a zero in a position, do not add the value.
  • If all 8 bits are zeros, 00000000, the value of the octet is 0.
  • If all 8 bits are ones, 11111111, the value of the octet is 255 (128+64+32+16+8+4+2+1).
  • If the 8 bits are mixed, such as the example 00100111, the value of the octet is 39 (32+4+2+1).
• The IPv4 Address Structure—The logical 32-bit IPv4 address is hierarchical and is made up of two parts. The first part identifies the network, and the second part identifies a host on that network. In hierarchical addressing, the network portion indicates the network on which each unique host address is located.

Logical AND is the comparison of two bits that produce results of either 0 or 1. In digital logic, 1 represents True and 0 represents False. When you‛re using an AND operation, both input values must be True (1) for the result to be True (1). Only a 1 AND 1 produce a 1. All other AND combinations produce a 0. To identify the network address of an IPv4 host, the IPv4 address is logically ANDed, bit by bit, with the subnet mask. ANDing between the address and the subnet mask yields the network address. The subnet mask is compared to the IPv4 address from left to right, bit for bit. The ones in the subnet mask represent the network portion; the zeros represent the host portion. A subnet mask of 255.255.255.0 (decimal) or 11111111.11111111.1111111.00000000 (binary) uses 24 bits to identify the network number, which leaves 8 bits to number the hosts on that network.

• Classful IPv4 Addressing—In 1981, Internet IPv4 addresses were assigned using classful addressing, based on one of three classes—A, B, or C:
  • Class A (0.0.0.0/8 to 127.0.0.0/8)—Designed to support extremely large networks with more than 16 million host addresses.
  • Class B (128.0.0.0 /16 to 191.255.0.0 /16)—Designed to support the needs of moderate to large size networks with up to approximately 65,000 host addresses.
  • Class C (192.0.0.0 /24 to 223.255.255.0 /24)—Designed to support small networks with a maximum of 254 hosts.

Classful addressing was abandoned in the late 1990s for the newer and current classless addressing system.

• Public and Private IPv4 Addresses—Most internal networks, from large enterprises to home networks, use private IPv4 addresses for addressing all internal devices (intranet) including hosts and routers. However, private addresses are not globally routable. Specifically, the private address blocks are
  • 10.0.0.0 /8 or 10.0.0.0 to 10.255.255.255
  • 172.16.0.0 /12 or 172.16.0.0 to 172.31.255.255
  • 192.168.0.0 /16 or 192.168.0.0 to 192.168.255.255

These addresses are not routable on the Internet. Before an ISP can forward a packet with a private address out to the Internet, the address must be translated to a public IPv4 address using NAT.

Public addresses (both IPv4 and IPv6) must be unique, and their use is regulated and allocated to each organization separately. Public addresses are managed by the IANA. The IANA manages and allocates blocks of IP addresses to the RIRs. RIRs are responsible for allocating IP addresses to ISPs, which, in turn, provide IPv4 address blocks to organizations and smaller ISPs.

• Unicast, Broadcast, and Multicast Addresses—For unicast communication, the addresses assigned to the two end devices are used as the source and destination IPv4 addresses. IPv4 unicast host addresses are in the address range of 0.0.0.0 to 223.255.255.255.

Broadcast traffic is used to send packets to all hosts on the network using the broadcast address for the network. With a broadcast, the packet contains a destination IPv4 address with all ones (1s) in the host portion. This means that all hosts on that local network (broadcast domain) will receive and look at the packet. Because routers separate broadcast domains, subdividing networks can improve network performance by eliminating excessive broadcast traffic.

Multicast transmission reduces traffic by allowing a host to send a single packet to a selected set of hosts that subscribe to a multicast group. The IPv4 multicast addresses 224.0.0.0 to 224.0.0.255 are reserved for multicasting on the local network only. Each multicast group is represented by a single IPv4 multicast destination address. When an IPv4 host subscribes to a multicast group, the host processes packets addressed to this multicast address and packets addressed to its uniquely allocated unicast address.

Calculate the Number of Hosts (8.3.7)–Cisco The Internet Protocol

The subnet masks seen most often with home and small business networking are 255.0.0.0 (8 bits), 255.255.0.0 (16 bits), and 255.255.255.0 (24 bits). A subnet mask of 255.255.255.0 (decimal) or 11111111.11111111.1111111.00000000 (binary) uses 24 bits to identify the network number, which leaves 8 bits to number the hosts on that network, as shown in Figure 8-6.

Figure 8-6 Calculating the Number of Hosts

To calculate the number of hosts that can be on that network, take the number 2 to the power of the number of host bits (28 = 256). From this number, you must subtract 2 (256–2). The reason you subtract 2 is that all ones within the host portion of an IPv4 address indicate a broadcast address for that network and cannot be assigned to a specific host. All zeros within the host portion indicate the network ID and, again, cannot be assigned to a specific host. Powers of 2 can be calculated easily with the calculator that comes with any Windows operating system.

Another way to determine the number of hosts available is to add up the values of the available host bits (128+64+32+16+8+4+2+1 = 255). From this number, subtract 1 (255–1 = 254), because the host bits cannot be all ones. It is not necessary to subtract 2 because the value of all zeros is 0 and is not included in the addition.

With a 16-bit mask, there are 16 bits (two octets) for host addresses, and a host address could have all ones (255) in one of the octets. This might appear to be a broadcast, but as long as the other octet is not all ones, it is a valid host address. Remember that the host looks at all host bits together, not at octet values.

Video—Network, Host, and Broadcast Addresses (8.3.8)

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Classful IPv4 Addressing (8.4)

Classful addressing is a legacy method of how IPv4 addresses were automatically assigned subnet masks based on the first several bits of the address. Although classful addressing has been made obsolete by classless addressing, it is important to understand the differences.

Classful and Classless Addressing (8.4.1)

In 1981, Internet IPv4 addresses were assigned using classful addressing. Customers were allocated a network address based on one of three classes—A, B, or C. The addresses were divided into the following ranges or classes:

• Class A (0.0.0.0/8 to 127.0.0.0/8)—Designed to support extremely large networks with more than 16 million host addresses. It used a fixed /8 prefix (255.0.0.0) with the first octet to indicate the network address and the remaining three octets for host addresses.
• Class B (128.0.0.0 /16 to 191.255.0.0 /16)—Designed to support the needs of moderate to large size networks with up to approximately 65,000 host addresses. It used a fixed /16 prefix (255.255.0.0) with the two high-order octets to indicate the network address and the remaining two octets for host addresses.
• Class C (192.0.0.0 /24 to 223.255.255.0 /24)—Designed to support small networks with a maximum of 254 hosts. It used a fixed /24 prefix (255.255.255.0) with the first three octets to indicate the network and the remaining octet for the host addresses.

Note

A Class D multicast block consists of 224.0.0.0 to 239.0.0.0, and a Class E experimental address block consists of 240.0.0.0 to 255.0.0.0.

As shown in Figure 8-7, the classful system allocated 50 percent of the available IPv4 addresses to 128 Class A networks, 25 percent of the addresses to Class B, and then Class C shared the remaining 25 percent with Classes D and E. Although appropriate at the time, as the Internet grew, it became obvious that this method was wasting addresses and depleting the number of available IPv4 network addresses.

Figure 8-7 Classful Addressing

Classful addressing was abandoned in the late 1990s for the newer and current classless addressing system. The formal name is classless interdomain routing (CIDR, pronounced “cider”). With classless addressing, customers receive an IPv4 network address and any size subnet mask, appropriate to the number of hosts required. The subnet mask can be any length and is not limited to the three subnet masks used in classful addressing.

Video—Classful IPv4 Addressing (8.4.2)

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