A subnet is a smaller network carved out of a larger block of IP addresses, in which every address shares the same network prefix. The remaining bits of each address identify individual hosts. A prefix length such as /24 shows how many leading bits are shared. Networks use subnets to organize their addresses and to control how traffic is routed.

Anti-bot systems can flag or rate-limit at the subnet level, not just per IP, so requests from different IPs in the same subnet can still be linked. This is a common reason proxy rotation can fail to avoid detection even when the individual IPs are technically different: if all the rotated IPs come from the same /24 subnet, a sophisticated anti-bot system can still correlate the traffic as coming from a single source. Providers with genuinely diverse pools distribute IPs across many different subnets and ASNs, not just many individual addresses, to avoid this failure mode.

A /24 subnet contains 256 addresses, all of which can be blocked together when a site bans an entire subnet.

Subnet Masks and CIDR Notation

  • Subnet mask: A 32-bit value whose leading 1 bits mark the network part of an IPv4 address. The mask 255.255.255.0 has 24 leading ones.
  • CIDR notation: Writes the same mask as a prefix length after a slash, so 255.255.255.0 becomes /24.
  • Block size: Each extra prefix bit halves the block. A /25 holds 128 addresses, a /23 holds 512, and a /32 is a single address.
  • IPv6: Standard IPv6 unicast addresses reserve the last 64 bits for the interface ID, so the subnet prefix is 64 bits long (a /64).

Why Networks Use Subnets

Organizations split one network into subnets when a single local network is not practical. RFC 950, the original subnetting standard, lists reasons such as mixing different LAN technologies, limits on the number of hosts and cable length, congestion caused by a few busy hosts, and sites joined by point-to-point links. Because routing protocols carry the prefix length, routers can treat one subnet, or a group of neighboring subnets, as a single route.