Packet Guide to Core Network Protocols

Chapter 7. Subnetting and Other Masking Acrobatics

“While these problems could be avoided by attempting to restrict the growth of the Internet, most people would prefer solutions that allow growth to continue. Fortunately, it appears that such solutions are possible, and that, in fact, our biggest problem is having too many possible solutions rather than too few.”

—RFC1380

A network can be defined in many ways. From a Layer-3 perspective, a network is a group of nodes that all share the same IP addressing scheme. The original vision for the IP-based Internet was a two-tier system in which a collection of networks were all connected to a single Internet or catenet. Confusion arises because it can be difficult to tell what the network boundaries are. The answer, and perhaps the source of the confusion, lies in the network mask. Many networking decisions are made based on the mask—host and router routing, classful and classless address space, security, QoS provisioning, and the overall design are all affected by the masks applied to the nodes.

A device operating on a network requires four numbers to ensure basic connectivity: IP address, network mask, gateway, and the DNS address. Their purpose is straightforward. IP addresses provide logical location, masks determine the network, the gateway is a router providing a pathway off of the current network, and the domain name server converts between IP addresses and more human-friendly addresses/words such as those used in web pages. The focus ...