15.8.2Subnetworks and subnet masks

We may check to see the IP addresses and subnet masks are correct by using the ping command. A screenshot of ping being used on a personal computer running the Microsoft Windows XP operating system is shown here:

In the next example, we see two computers with the same mask value, but with di erent address values in the octets designated by their masks. In other words, these two computers belong to di erent subnets: one to 167.254 and the other to 169.254, and as a result they are not allowed to communicate with each other using Internet Protocol. The resulting error messages generated by the ping utility are shown in this diagram:

In the last example, we see two computers having di erent mask values as well as di erent IP addresses. The subnet of the left-hand computer is 169.254.10 while the subnet of the right-hand computer is 169.254:

Computers with different subnet masks, on different subnets

The computer on the left may only communicate with IP addresses matching in the first three octets (169.254.10). Seeing that the destination address for the second computer does not match in its third octet, ping returns a “Destination host unreachable” error message when executed from the left-hand computer.

When the computer on the right attempts to communicate with (“ping”) the computer on the left, it is allowed to transmit to that computer because its mask only screens for agreement in the first two octets (169.254), which happen to match. However, the computer on the left is not allowed to transmit to the computer on the right because of its more restrictive subnet, and so ping running on the right-hand computer returns a “Request timed out” error message because it never receives a reply from the left-hand computer to any of its queries.

With just two computers connected by a single cable, the concept of subnetworks and masks seems useless, and indeed it is on this small of a scale. However, “subnetting” is a useful technique for managing high tra c loads on large networked systems using IP addresses, and so it is commonly seen in many local area networks (LANs) such as those found at industry and commercial sites.

While many IPv4-compliant computers designate both the IP address and the subnet mask values as sets of “dotted-decimal” numbers with each decimal (0-255) representing an “octet” of eight bits in the 32-bit IPv4 address space (e.g. IP = 169.254.5.1 and Mask = 255.255.0.0), a more modern designation for subnets is to append the device’s IP address with a forward slash character and a decimal number specifying how many bits are used to specify the subnet.

To illustrate by example, consider a computer having the IP address 169.254.5.1 and a mask value of 255.255.0.0 (thus specifying that it belongs to the subnetwork 169.254), we could alternatively state that computer’s IP address as 169.254.5.1/16. The “16” means that the first sixteen bits of its IP address define its subnet. To cite another example, a computer with an IP address of 192.168.35.100 and a subnet mask of 255.255.255.0 could have its address alternatively written as 192.168.35.100/24. The “24” tells us the first 24 bits (first three octets) of the IP address define its subnetwork: this computer belongs to the subnetwork 192.168.35.

The ping diagnostic utility program may be used to search for unknown IP addresses on a known subnet. This may be done by “pinging” to the broadcast address for that subnet: an IP address formed by the known subnet numbers, followed by all binary 1’s filling the unknown bit spaces. For example, you could use ping to search for devices on the subnet 156.71 (subnet mask 255.255.0.0) by using the following command:

ping 156.71.255.255