Finding Subnet Information Given a Host Address and the Mask
Given a host address and the subnet mask, you can determine the subnet on which that host lives. This is another common exercise and is useful anytime you need to track the subnet number for a host (in a routing table, for example). Suppose you are given the following host address and subnet mask:
172.16.9.136/22
To start the process, convert the host address and mask to binary and write the mask below the host address (for clarity, the host field bits are printed in boldface here):
1010-1100.0001-0000.0000-1001.1000-1000 = 172.16.9.136 1111-1111.1111-1111.1111-1100.0000-0000 - /22
Now, focus on the boundary defined by the mask (where the ones end and the zeros begin). This is the boundary between the subnet field and the host field and tells you that the last 10 bits of the address make up the host field. An easy way to determine the subnet number is to take the host address and set all of the bits in the host field to zero, like this:
1010-1100.0001-0000.0000-1000.0000-0000 = 172.16.8.0 Thus, host 172.16.9.136/22 is on subnet 172.16.8.0/22.
NOTE You might notice that the subnet number is the result of a binary "AND" operation on the address and mask at each bit position. This is how computers (and routers) calculate the subnet number.
Additionally, you can easily find the IP broadcast address for the subnet. This is done by setting all of the bits in the host field (printed again in boldface) to one, like this:
1010-1100.0001-0000.0000-1011.1111-1111 = 172.16.11.255
Thus, the broadcast address of subnet 172.16.8.0/22 is 172.16.11.255. Sending a packet (a ping, for example) to 172.16.11.255 is a transmission to every host in the subnet.
Last, you can find the range of valid host addresses for this subnet. The range contains the addresses between the subnet number (host field of all zeros) and the broadcast address (host field of all ones), so the host address range for subnet 172.16.8.0/22 is
1010-1100.0001-0000.0000-1000.0000-0001 = 172.16.8.1
through
1010-1100.0001-0000.0000-1011.1111-1110 = 172.16.11.254
You can verify that the host address 172.16.9.136. introduced at the start of this section, indeed falls within this address range.
Note that subnetting is restrictive because the technique forces you to commit to the number of subnets you need now and in the future. You also need to commit to the number of hosts per subnet, because every bit you steal for the subnet field means one less bit you can use for host addresses.
Making matters worse, the technique produces subnets that are all of equal size in the number of hosts that can be supported per subnet. Therefore, you often have to do the sizing based on the largest subnet needed and waste addresses when deploying the remaining subnets to areas with fewer hosts. These issues apply when you're using a routing protocol that only supports a fixed-size mask. "Subnetting with Variable Length Subnet Masks," later in this chapter, covers a method of subnetting that mitigates some of the problems with fixed-size masks.
The Rules on Top and Bottom Subnets
Arguments exist both in theory and in practice for not using the top and bottom subnets in a classful network. Theoretically, a bit field has two special patterns:
• All-zeros pattern— usually means "this" as in "this host" or "this network."
• All-ones pattern—usually means "all" as in "all hosts" or "all networks."
Early internet documents said it was a good idea to keep these meanings and apply them to the subnet field, thus disallowing the use of the bottom subnet of all zeros and the top subnet of all ones. As a result, IP software in devices obeyed these rules and checked if users erroneously attempted to configure a device in violation of the rules.
NOTE i nc advent of classless addressing abolished the notion of the top and bottom subnets (and subnets in general). In a classless environment, devices can use the address space that the classful world knows as the lop and bottom subnets. See "Overview of Classless Addressing" later in this chapter for information on classless addressing.
In practice, using the top or bottom subnet can be problematic, because not all devices, especially legacy devices, allow these to be configured. Although you might be successful at deploying some hosts and routers on these outer subnets, you might find that other devices forbid you to configure an address from the top or bottom subnet. You'll then have to find another subnet for those devices. To avoid problems, a good idea is to be familiar with the diversity of devices in your environment and determine the addressing allowed on those devices.
Disadvantages of Subnetting
The root of the controversy lies in the ambiguity of addresses when you're using the top or bottom subnets. Take, for example, a bottom subnet field that contains all zeros (the host field also contains all zeros)—the subnet number is the same as the major net number. This is apparent in Example 1-1, where the bottom subnet 192.168.1.0/27 is the same address as the major net (see Table 1-2). This ambiguity can be a source of confusion for some devices because a reference to the subnet is indistinguishable from a reference to the major net. Similarly, an all-ones broadcast to the top subnet could be interpreted as a broadcast address to all of the major net, because the top subnet and major net broadcasts are also indistinguishable. Looking again at the example in Table 1 -2, a broadcast to the upper subnet 192.168.1.224/27 is 192.168.1.255—the same address as a broadcast to the entire class C (192.168.1.0).
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