3.7 A Subnet Example

This example shows the subnet used in the text, and how two different subnet masks are used. Figure 3.10 shows the arrangement.

Figure 3.10. Arrangement of hosts and networks for author's subnet.

If you compare this figure with the one on the inside front cover, you'll notice that we've omitted the detail that the connection from the router sun to the top Ethernet in Figure 3.10 is really a dialup SLIP connection. This detail doesn't affect our description of subnetting in this section. We'll return to this detail in Section 4.6 when we describe proxy ARP.

The problem is that we have two separate networks within subnet 13: an Ethernet and a point-to-point link (the hardwired SLIP link). (Point-to-point links always cause problems since each end normally requires an IP address.) There could be more hosts and networks in the future, but not enough hosts across the different networks to justify using another subnet number. Our solution is to extend the subnet ID from 8 to 11 bits, and decrease the host ID from 8 to 5 bits. This is called variable-length subnets since most networks within the 140.252 network use an 8-bit subnet mask while our network uses an 11-bit subnet mask.

RFC 1009 [Braden and Postel 1987] allows a subnetted network to use more than one subnet mask. The new Router Requirements RFC [Almquist 1993] requires support for this.

The problem, however, is that not all routing protocols exchange the subnet mask along with the destination network ID. We'll see in Chapter 10 that RIP does not support variable-length subnets, while RIP Version 2 and OSPF do. We don't have a problem with our example, since RIP isn't required on the author's subnet.

Figure 3.11 shows the IP address structure used within the author's subnet. The first 8 bits of the 11-bit subnet ID are always 13 within the author's subnet. For the remaining 3 bits of the subnet ID, we use binary 001 for the Ethernet, and binary 010 for the point-to-point SLIP link. This variable-length subnet mask does not cause a problem for other hosts and routers in the 140.252 network ”as long as all datagrams destined for the subnet 140.252.13 are sent to the router sun (IP address 140.252.1.29) in Figure 3.10, and if sun knows about the 11-bit subnet ID for the hosts on its subnet 13, everything is fine.

Figure 3.11. Using variable-length subnets.

The subnet mask for all the interfaces on the 140.252.13 subnet is 255.255.255.224, or 0xffffffe0. This indicates that the rightmost 5 bits are for the host ID, and the 27 bits to the left are the network ID and subnet ID.

Figure 3.12 shows the allocation of IP addresses and subnet masks for the interfaces shown in Figure 3.10.

Figure 3.12. IP addresses on author's subnet.

The first column is labeled "Host," but both sun and bsdi also act as routers, since they are multihomed and route packets from one interface to another.

The final row in this table notes that the broadcast address for the bottom Ethernet in Figure 3.10 is 140.252.13.63: it is formed from the subnet ID of the Ethernet (140.252.13.32) and the low-order 5 bits in Figure 3.11 set to 1 (16 + 8 + 4 + 2 + 1 = 31). (We'll see in Chapter 12 that this address is called the subnet-directed broadcast address.)