CFWorx

Reserved IPv4 addresses

· in Cisco U

Some IPv4 addresses can never be handed to a host. Quick reference for the reserved ones.

Network address

All host bits set to 0: it names the network itself. 10.0.0.0 is the network holding host 10.1.2.3; 172.16.0.0 is a Class B network address; 192.168.1.0 is Class C.

Routers match destination networks against these in the routing table. The same rule holds after subnetting: 172.16.1.0/24 is a subnet address, not a usable host.

Local broadcast: 255.255.255.255

The destination for “everyone on my local segment.” The classic use is a host with no address yet shouting for a DHCP server.

Routers never forward it. It dies at the first router.

Directed broadcast

All host bits set to 1: the highest address in the network, reaching every host in that specific network.

Unlike the local broadcast, this one can be routed toward the target network. That routability enabled the 1990s Smurf DoS attack: flood a victim by pinging directed broadcasts.

Since IOS 12.0 the platform default is no ip directed-broadcast, so routers drop them at the final hop. Re-enable per interface with ip directed-broadcast only with a real reason.

On a hypothetical fully populated 10.0.0.0/8, pinging 10.255.255.255 would draw 16,777,214 replies.

Loopback: 127.0.0.0/8

The device talking to itself. Usually 127.0.0.1, but any 127.x.x.x address works.

Ping it to test the local TCP/IP stack, no NIC or cable involved. It also lets apps on the same box talk over TCP/IP.

Link-local / APIPA: 169.254.0.0/16

Self-assigned (RFC 3927) when a host finds no static config and DHCP fails. It works only on the local segment and is never routed.

Seeing 169.254.x.x on a PC almost always means “DHCP broke.” Treat it as a failure symptom.

Microsoft calls the feature APIPA; Apple implements it too.

Documentation blocks

Reserved for docs and example code, never on the real internet: 192.0.2.0/24, 198.51.100.0/24, and 203.0.113.0/24 (RFC 5737). They pair with example.com and example.net in vendor docs.

All zeros: 0.0.0.0

Means “this host on this network,” and it’s a source address only. Seen during DHCP: the client sources from 0.0.0.0 before it has a lease.

The full reserved list lives in RFC 5735 (since superseded by RFC 6890).

Related: public and private IPv4 addresses, IPv4 structure and classes.