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Networking

CIDR notation

Classless Inter-Domain Routing notation, a shorthand for subnet masks. /24 means the first 24 bits are the network, leaving 8 bits for hosts. /24 gives 254 usable IPv4 addresses.

Reading the number after the slash

In 192.168.1.0/24, the number after the slash counts the bits at the start of the address that name the network. An IPv4 address has 32 bits, so whatever's left over belongs to hosts. A /24 leaves 8 host bits, which is 256 addresses. A /26 leaves 6 bits and 64 addresses.

Each extra bit in the prefix cuts a block in half. A /24 splits into 2 blocks of /25 or 4 blocks of /26. The slash is also the subnet mask in short form: 255.255.255.0 is 24 one bits followed by 8 zeros, so it's /24.

Sizes worth knowing by heart

These cover most real networks. The usable count is the total minus the network and broadcast addresses.

  • /30: 4 addresses, 2 usable, mask 255.255.255.252. A link between two routers.
  • /28: 16 addresses, 14 usable, mask 255.255.255.240. A small server group.
  • /24: 256 addresses, 254 usable, mask 255.255.255.0. A typical office or home LAN.
  • /22: 1,024 addresses, 1,022 usable, mask 255.255.252.0. A busy site.
  • /16: 65,536 addresses, 65,534 usable, mask 255.255.0.0. A whole campus or a cloud network.

Type any prefix into the Subnet Calculator to see its mask, host range and count.

What CIDR replaced

Before CIDR, a block was a Class A (/8), a Class B (/16) or a Class C (/24), with nothing in between. A company with 2,000 devices was too big for a Class C, so it took a Class B and used a small slice of 65,536 addresses. RFC 4632 describes the pressure that ended this: Class B space was running out, and routing tables were growing beyond what routers could manage.

CIDR was built to address those two problems. A block can be any power of two, so that company fits in a /21 with 2,046 usable addresses. A provider can also announce one aggregate route for a whole block of customers instead of one route per customer. CIDR didn't fix running out of IPv4 addresses. That's what IPv6 is for.

Limits you hit in the cloud

AWS allows IPv4 subnets from /16 to /28, and Azure allows /2 to /29. Both keep five addresses in every subnet: the first four and the last one. A /28 has 16 addresses but only 11 for your resources, and a /24 gives 251.

That's why people round up. For 100 servers, a /25 leaves 123 usable addresses in the cloud, since a /26 gives only 59.

Work it out yourselfOpen the Subnet Calculator

Read next

What a /24 subnet fits When a /22 fits CIDR cheat sheet for VPC and LAN design

Related terms

Subnet maskA 32-bit number that divides an IP address into network and host portions. 255.255.255.0 is the mask for a /24 subnet, meaning the last octet is available for host addresses.Wildcard maskA 32-bit pattern written like an IPv4 address. A 0 bit means the address bit must match and a 1 bit means it's ignored. It's the inverse of a subnet mask, and Cisco style access lists and OSPF network statements use it.Broadcast addressThe last IP address in an IPv4 subnet, used to send data to all devices on that network segment simultaneously. IPv6 has no broadcast, using multicast instead.IPv6The current version of the Internet Protocol, with 128-bit addresses instead of IPv4's 32-bit ones. A LAN is normally a /64, and there's no broadcast address, since multicast does that job.

Sources

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