3.4 Subnet Addressing

3.4 Subnet Addressing

All hosts are now required to support subnet addressing (RFC 950 [Mogul and Postel 1985]). Instead of considering an IP address as just a network ID and host ID, the host ID portion is divided into a subnet ID and a host ID.

This makes sense because class A and class B addresses have too many bits allocated for the host ID: 2 ²⁴ “ 2 and 2 ¹⁶ “ 2, respectively. People don't attach that many hosts to a single network. (Figure 1.5 shows the format of the different classes of IP addresses.) We subtract 2 in these expressions because host IDs of all zero bits or all one bits are invalid.

After obtaining an IP network ID of a certain class from the InterNIC, it is up to the local system administrator whether to subnet or not, and if so, how many bits to allocate to the subnet ID and host ID. For example, the internet used in this text has a class B network address (140.252) and of the remaining 16 bits, 8 are for the subnet ID and 8 for the host ID. This is shown in Figure 3.5.

Figure 3.5. Subnetting a class B address.

This division allows 254 subnets, with 254 hosts per subnet.

Many administrators use the natural 8-bit boundary in the 16 bits of a class B host ID as the subnet boundary. This makes it easier to determine the subnet ID from a dotted -decimal number, but there is no requirement that the subnet boundary for a class A or class B address be on a byte boundary.

Most examples of subnetting describe it using a class B address. Subnetting is also allowed for a class C address, but there are fewer bits to work with. Subnetting is rarely shown with a class A address because there are so few class A addresses. (Most class A addresses are, however, subnetted .)

Subnetting hides the details of internal network organization (within a company or campus) to external routers. Using our example network, all IP addresses have the class B network ID of 140.252. But there are more than 30 subnets and more than 400 hosts distributed over those subnets. A single router provides the connection to the Internet, as shown in Figure 3.6.

Figure 3.6. Arrangement of most of the noao.edu 140.252 subnets.

In this figure we have labeled most of the routers as R n, where n is the subnet number. We show the routers that connect these subnets, along with the nine systems from the figure on the inside front cover. The Ethernets are shown as thicker lines, and the point-to-point links as dashed lines. We do not show all the hosts on the various subnets. For example, there are more than 50 hosts on the 140.252.3 subnet, and more than 100 on the 140.252.1 subnet.

The advantage to using a single class B address with 30 subnets, compared to 30 class C addresses, is that subnetting reduces the size of the Internet's routing tables. The fact that the class B address 140.252 is subnetted is transparent to all Internet routers other than the ones within the 140.252 subnet. To reach any host whose IP address begins with 140.252, the external routers only need to know the path to the IP address 140.252.104.1. This means that only one routing table entry is needed for all the 140.252 networks, instead of 30 entries if 30 class C addresses were used. Subnetting, therefore, reduces the size of routing tables. (In Section 10.8 we'll look at a new technique that helps reduce the size of routing tables even if class C addresses are used.)

To show that subnetting is not transparent to routers within the subnet, assume in Figure 3.6 that a datagram arrives at gateway from the Internet with a destination address of 140.252.57.1. The router gateway needs to know that the subnet number is 57, and that datagrams for this subnet are sent to kpno. Similarly kpno must send the datagram to R55, who then sends it to R57.