This chapter covers
- What subnetting is and why it’s necessary
- How to borrow bits from the host portion of a network to expand the network portion and create subnets
- How to identify the five attributes of a subnet
- How to divide a network into subnets of equal and variable sizes
In chapter 7, we covered IPv4 address classes, focusing on classes A, B, and C—the three classes of addresses which can be assigned to hosts. Each class is defined by the first bit(s) of the address, and the prefix length of addresses in each range is also defined:
- Class A addresses begin with 0b0 and use a /8 prefix length.
- Class B addresses begin with 0b10 and use a /16 prefix length.
- Class C addresses begin with 0b110 and use a /24 prefix length.
This addressing architecture, called classful addressing, was defined in the original Internal Protocol standard in 1981 (RFC 791). However, with the rapid growth of the internet, classful addressing soon proved to be too rigid, resulting in inefficient use of addresses; the pool of available IPv4 addresses was drying up. Subnetting, which involves dividing a larger network up into smaller networks, is one answer to this problem and is a fundamental skill for network engineers. Subnetting is the second half of CCNA exam topic 1.6: Configure and verify IPv4 addressing and subnetting.