3.5 Subnet Mask

3.5 Subnet Mask

Part of the configuration of any host that takes place at bootstrap time is the specification of the host's IP address. Most systems have this stored in a disk file that's read at bootstrap time, and we'll see in Chapter 5 how a diskless system can also find out its IP address when it's bootstrapped.

In addition to the IP address, a host also needs to know how many bits are to be used for the subnet ID and how many bits are for the host ID. This is also specified at bootstrap time using a subnet mask. This mask is a 32-bit value containing one bits for the network ID and subnet ID, and zero bits for the host ID. Figure 3.7 shows the formation of the subnet mask for two different partitions of a class B address. The top example is the partitioning used at noao.edu, shown in Figure 3.5, where the subnet ID and host ID are both 8 bits wide. The lower example shows a class B address partitioned for a 10-bit subnet ID and a 6-bit host ID.

Figure 3.7. Example subnet masks for two different class B subnet arrangements.

Although IP addresses are normally written in dotted -decimal notation, subnet masks are often written in hexadecimal, especially if the boundary is not a byte boundary, since the subnet mask is a bit mask.

Given its own IP address and its subnet mask, a host can determine if an IP datagram is destined for (1) a host on its own subnet, (2) a host on a different subnet on its own network, or (3) a host on a different network. Knowing your own IP address tells you whether you have a class A, B, or C address (from the high-order bits), which tells you where the boundary is between the network ID and the subnet ID. The subnet mask then tells you where the boundary is between the subnet ID and the host ID.

Example

Assume our host address is 140.252.1.1 (a class B address) and our subnet mask is 255.255.255.0 (8 bits for the subnet ID and 8 bits for the host ID).

• If a destination IP address is 140.252.4.5, we know that the class B network IDs are the same (140.252), but the subnet IDs are different (1 and 4). Figure 3.8 shows how this comparison of two IP addresses is done, using the subnet mask.

  Figure 3.8. Comparison of two class B addresses using a subnet mask.

  

• If the destination IP address is 140.252.1.22, the class B network IDs are the same (140.252), and the subnet IDs are the same (1). The host IDs, however, are different.

• If the destination IP address is 192.43.235.6 (a class C address), the network IDs are different. No further comparisons can be made against this address.

The IP routing function makes comparisons like this all the time, given two IP addresses and a subnet mask.