What does /30 mean?

Answer

A /30 has 4 addresses and exactly 2 usable hosts, with netmask 255.255.255.252. In 198.51.100.0/30, .0 is the network, .3 the broadcast, and .1 and .2 are the two ends of the link.

Worked example: 198.51.100.0/30. Type any block to compare.

198.51.100.0/30 is 4 addresses: 198.51.100.0 to 198.51.100.3. 2 usable for hosts (the first is the network address, the last is broadcast).

19811000110510011001110001100100000000000

30 network bits 2 host bits

Network: 198.51.100.0
First usable: 198.51.100.1
Last usable: 198.51.100.2
Broadcast: 198.51.100.3
Addresses: 4
Usable hosts: 2
Netmask: 255.255.255.252
Wildcard (ACL) mask: 0.0.0.3
Historic class: C
Range type: Documentation (TEST-NET-2) · not routed on the internet

Special-purpose range

Documentation (TEST-NET-2) 198.51.100.0/24 RFC 5737
Reserved for examples in documentation.

Split it into subnets

4 subnets.

Subnet	Usable range	Hosts	Broadcast
`198.51.100.0/32`	`198.51.100.0` – `198.51.100.0`	1	`–`
`198.51.100.1/32`	`198.51.100.1` – `198.51.100.1`	1	`–`
`198.51.100.2/32`	`198.51.100.2` – `198.51.100.2`	1	`–`
`198.51.100.3/32`	`198.51.100.3` – `198.51.100.3`	1	`–`

Is this IP in the range?

Range to CIDR blocks

What to know

Two usable hosts is exactly what a point-to-point link needs: one address for each router. For decades /30 was the standard for WAN links and ISP-to-customer handoffs.

Half of a /30 goes to the network and broadcast addresses, which a two-host link does not need. Most modern routers support /31 links, which use both addresses.

Gotchas

Some cloud VPN and interconnect services require link addresses from 169.254.0.0/16 in /30 blocks (AWS Site-to-Site VPN tunnel inside addresses, for example). Use the exact range the service asks for.
Both ends must use the same /30. A typo that puts one side in an adjacent /30 makes the link fail silently.

Related explainers

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