Networking: A Beginner's Guide
Because subnet masks are usually created using contiguous bits for the mask itself,
only nine subnet masks are commonly used, as shown in Table 8-1.
In Table 8-1, some configurations are marked as N/A, for not applicable. These
subnet masks would result in no available addresses, because of the rule that the
subnet portion of the netid cannot be all 0s or all 1s. For example, consider the subnet
mask of 224, which uses three hostid bits for the subnetid. In theory, this configuration
should result in eight subnets. However, the subnets represented by 000 and 111 are not
valid. Likewise, 128 is not a valid subnet mask because that one bit would always be
either a 1 or a 0.
If you need to implement subnets, you should initially work through the project with an
experienced network engineer, who can help you avoid pitfalls (which were not explicitly described
in the preceding section). You might also want to learn more about TCP/IP through resources
devoted to detailed coverage of the concepts introduced here.
Understanding Other Internet Protocols
Quite a few other protocols used on the Internet either rely on or make use of TCP/IP.
In this section, you learn about these different protocols.
Domain Name System (DNS)
If you had only IP address numbers to address computers over the Internet, trying
to keep track of them and using their correct addresses might make you a little crazy.
To go to the web site for Google, for example, you would need to remember to type
Most Common Subnet Masks
Number of Subnets
Number of Hostids per Subnet