The /30 prefix: 4 addresses and a 50% tax
A /30 keeps only 2 host bits: 4 addresses, mask 255.255.255.252. Two of those four are spent immediately — network and broadcast — leaving exactly 2 usable addresses, one for each end of a link.
For decades this was the default point-to-point subnet: every serial link, every ISP handoff, every router-to-router interconnect got its own /30. It works, but half the block is overhead, which is why RFC 3021's /31 exists — and why /30 is increasingly a legacy habit.
| Prefix length | /30 |
| Subnet mask | 255.255.255.252 |
| Wildcard mask | 0.0.0.3 |
| Total addresses | 4 |
| Usable hosts | 2 |
The 50% overhead problem
No other common subnet size wastes this much. A /24 loses under 1% of its addresses to network and broadcast; a /30 loses half. On a router-rich topology with hundreds of point-to-point links, /30 addressing burns two extra addresses per link — a real cost when links are numbered out of a constrained block.
Where /30s still make sense
Legacy gear that predates RFC 3021 support is the honest reason: some platforms will not accept a /31 on an interface, so the /30 remains the safe common denominator. ISP handoffs are the other holdout — plenty of providers still provision customer interconnects as /30s out of habit and tooling, and there is no pressing reason to argue with them.
CIDR Calculator
Run the math on 10.0.0.0/30 — or any other block
Frequently asked questions
How many IP addresses are in a /30?
4 in total (2^2), with 2 usable — the network and broadcast addresses consume the other half.
What is the subnet mask of a /30?
255.255.255.252. Six network bits in the last octet, so /30 networks begin on multiples of 4.
Why is a /30 used for point-to-point links?
It provides exactly two host addresses, one per router, while still following the classic network-plus-broadcast rules that older equipment expects.
Should I use a /30 or a /31 for a new link?
Use a /31 (RFC 3021) if both ends support it — it wastes no addresses. Keep /30 for legacy devices or providers that require traditional subnet semantics.