What is classlless subnetting?

Answer 1

Classful addressing (RFC 791) was the internet's first major addressing scheme. The IP address was 32 bits in size, just as today, but was initially managed differently. There were three address classes to chose from: A, B, or C, corresponding to 8-bit, 16-bit, or 24-bit prefixes. No other prefix lengths were allowed, and there was no concept of nesting a group of 24-bit prefixes, for example, within a 16-bit prefix. An address was slotted into one of three address classes based on its high-order bits. Addresses beginning with 0 were considered class A; addresses beginning 10 were class B; addresses beginning 110 class C. Two other classes were also defined, class D addresses beginning 1110 and class E addresses beginning 1111, though neither of these two address classes were normally used. First Octet Address Class 0-127 Class A (/8)

128-191 Class B (/16)

192-223 Class C (/24)

224-239 Class D 240-255 Class E

A Class A (/8) contains 16777214 addresses, a Class B (/16) contains 65534 and a Class C (/24) contains 254.

Due to the explosive growth of the internet, this crude method of address distribution became too wasteful and would have rapidly led to the exhaustion of global IP space.

Classless subnetting (CIDR) is where subnets are created on arbitrary boundaries. CIDR allowed networks to be created and globally routed on arbitrary boundaries, for example an organisation needing 1000 addresses could be assigned a /22 (1024 addresses), rather than 4 Class Cs, or as was often the case, a Class B!

Within their /22 this organisation could then split the network up into even smaller "classless" segments, such as /30s (2 usable addresses) for point to point links or /27s (32 addresses, 30 usable) for small networks with up to 29 hosts.