Class C subnet

The Internet Protocol standard of January 1980 used the first byte of an address for the network and the other three for the machine on it. That left room for only 256 networks. The version of September 1981 replaced that with classes, in which the first bits of an address said where the network part ended. In class A the first 8 bits named the network and the last 24 the machine. Class B split 16 and 16. Class C used 24 bits for the network and left 8, enough for 254 hosts.

The sizes were a guess about the future. The 2006 CIDR standard recalls that "certain assumptions were made about the number of organizations to be connected" and about how many machines each would have. A multicast proposal of December 1985 suggested class D, then meaning every address starting with 111. The multicast standard proposed in July 1986 narrowed it to addresses starting 1110 and named those starting 1111 class E, "reserved for future addressing modes". Class E is still reserved today.

The class names were published in September 1981. The protocol text wrote them in lower case: class a, b and c. The list of assigned numbers published the same month labelled its diagrams Class A, Class B and Class C Address. That list wrote a class C network as nnn.nnn.nnn.rrr: three numbers for the network and the last one for the machine.

"Subnet" came later. A proposal of October 1984 and the subnetting standard of August 1985 let a site divide one class network into several subnets. The standard's example for a class C network gives 192.1.127.0 a three-bit subnet field, which makes eight subnets of 32 addresses. In that original sense a class C subnet was a piece of a class C network. Sellers now use the phrase for the whole /24.

The first CIDR document, in June 1992, called the idea supernetting. Each network provider would get a block of class C networks that the rest of the internet could reach through one route. It was meant as a stopgap, "viable for at least three (3) years". The 2006 CIDR standard notes that the old class C space that cannot be combined this way is sometimes called "the swamp". It put the swamp at about 15,000 routing table entries.

Most organisations fell between the sizes. The CIDR proposal of September 1993 said so directly. Class C, "with a maximum of 254 host addresses, is too small, while class B, which allows up to 65534 addresses, is too large for most organizations". Class B space was running out, and the routing tables were growing "beyond the ability of current software, hardware, and people to effectively manage".

The IETF had formed a group to study the problem in November 1991. Its answer, CIDR, dropped the classes. A block can be any power-of-two size, so a network gets a block close to what it needs. The size is written as a prefix length after a slash. Routing followed in 1994, when providers deployed BGP4 and the number of routing table entries dropped sharply. By June 1995 the standard for routers said the class of an address was of "only historical interest".

One change from May 1993 still shows in addresses today. The registry plan of that month divided part of the class C space by region. Europe got 194 and 195, North America 198 and 199, Central and South America 200 and 201, and the Pacific Rim 202 and 203. The IANA register still lists those numbers under the matching regional registries, and the proxies on our free list still follow the plan closely.

The class is gone, but the size is not. A /24 is the smallest IPv4 block that networks accept from each other. An IETF guide of 2015, citing the RIPE community, notes that longer prefixes "are generally neither announced nor accepted in the Internet". So a /24 is routed as one unit, normally by one network. On our free list, every one of the 411 blocks holding more than one live proxy sat in a single network.

Sites act on that. Cloudflare's IP access rules, for example, match a single address, an IPv4 range of /24 or /16, or a whole ASN. One rule covers every address in a /24, and one more covers every address the network holds.

A pool spread over many /24 blocks survives a block on any one of them, and that is what the count is meant to show. One seller advertised its datacenter proxies in August 2021 as 20,000 class C subnets in 9 ASNs. That pool is wide across blocks and narrow across networks: a site that blocks by ASN needs nine rules for it, not twenty thousand.

Our free proxy list, which collects public proxies from across the internet, has the opposite shape. On 19 September 2026 its 3,662 live IPv4 addresses sat in 2,920 different /24 blocks and 1,438 networks. Only 411 blocks held more than one address, and the busiest held 14.

When a seller quotes a class C count, ask for the number of networks as well. A spread over many /24 blocks protects a pool from a block on one range. Only a spread over many networks protects it from a block on a whole network.