CIDR

Classless Inter-Domain Routing represents an address range with a prefix length instead of relying on historical address classes. CIDR supports variable-sized allocations, subnetting, and route aggregation for IPv4 and IPv6.

Reading Prefix Notation

In 10.42.3.17/24, the first 24 bits are the network prefix and eight bits remain for positions within the range. The canonical network is 10.42.3.0/24; the supplied host address can still be written with the prefix when configuring an interface.

Calculating Range Size

IPv4 prefix /23 leaves nine host bits and therefore covers 2^9 = 512 total addresses. The aligned prefix 123.12.24.0/23 spans:

first: 123.12.24.0
last:  123.12.25.255

In traditional broadcast use, the first is the network address and the last the directed broadcast. Do not apply the “minus two” usable-host shortcut blindly to /31 point-to-point or /32 host routes.

Checking Alignment

A prefix must begin on its binary boundary. A /23 advances in blocks of two in the third octet when earlier octets are fixed, so 123.12.24.0/23 is aligned but 123.12.25.0/23 canonicalizes to the same 123.12.24.0/23 range.

Aggregating Routes

CIDR can advertise one aggregate for several contiguous, equally sized, correctly aligned prefixes. For example, 192.0.2.0/25 and 192.0.2.128/25 combine into 192.0.2.0/24. Aggregation is safe only when the advertising router can correctly reach the complete aggregate or has policy to prevent loops and black holes.

Longest-Prefix Routing

When routes overlap, forwarding normally selects the eligible route with the longest matching prefix. A /24 route is more specific than a covering /16, while a default route /0 matches only when no more-specific eligible route wins.

Summary

You can now use CIDR notation for both address ranges and route selection.

1. Interpret the slash value as a leading prefix-bit count.
2. Calculate total range size from remaining bits.
3. Canonicalize a prefix to its aligned network boundary.
4. Aggregate only contiguous aligned ranges with valid reachability.
5. Prefer the longest eligible prefix during route lookup.