IP networks and subnets

Before moving on, a short review of typical IP subnetting terms and typical applications should help clarify the terms used in this book and will act as a refresher for those already versed in IP addressing.

A /24 designator placed after a network IP address in diagrams or device configurations is a Classless Inter-Domain Routing (CIDR) designator that indicates the following:

• The first 24 out of the 32 bits in the 4-byte IP address represents the network portion of any IP address on this network. This network is designated as 10.1.1.0 (the next /24 network would be 10.1.2.0, then 10.1.3.0, and so on).
• The last 8 bits of the 32-bit address can be used to give workstations, hosts, and other devices an IP address, with the following exceptions:
  • The first host address on this network is reserved as a network designator to build routing tables: 10.1.1.0 (typically called the loopback address)
  • The last host address on this network is reserved as an IP broadcast address: 10.1.1.255

  The 8 bits binary is equal to 256 decimal, minus the preceding two exceptions. This leaves 254 usable IP addresses for devices, starting with 10.1.1.1, 10.1.1.2, and so on up to 10.1.1.254.

• Another way of expressing subnet masks is in a dotted decimal format, 255.255.255.0, which again indicates that the first 24 bits of an IP address is the network and the remaining 8 bits are for hosts.
• There are Class A, Class B, and Class C address ranges, as well as a subset of IP ranges reserved as private addresses to use within organizations.

  The following table shows the IP address ranges in the three major classes:

  

• Subnet masks can be configured to allow more or fewer hosts per subnet with a corresponding tradeoff in having fewer or greater network addresses with which to build multiple networks within larger organizations.

A deeper review of IP addressing and subnetting is beyond the scope of this book. If you're not familiar with these concepts, some additional study would be advisable as a solid understanding of IP subnetting is essential for most analysis activities.