The /30 Subnet: Point-to-Point Links Between Routers Explained
A /30 subnet provides exactly 4 IP addresses — 2 usable hosts plus a network and broadcast address. It is the traditional choice for WAN point-to-point links between two routers.
When two routers need to exchange routes across a WAN link, you need a subnet that connects exactly two devices. The /30 subnet has long been the standard choice — it provides 4 total addresses, 2 of which are usable hosts. The other two are the network address and the broadcast address, which are required by the IPv4 specification.
The /30 Address Layout
Take 192.168.1.0/30 as an example. Open it in the CIDR calculator and you will see:
192.168.1.0— Network address (reserved)192.168.1.1— First usable host (typically Router A)192.168.1.2— Second usable host (typically Router B)192.168.1.3— Broadcast address (reserved)
Router A is configured with 192.168.1.1/30 and Router B with 192.168.1.2/30. Each router knows the other is on the same /30 subnet and can send packets directly without a gateway.
Why Not Use a /29 or Larger?
A /29 gives 6 usable hosts — four more than needed for a two-router link. On a WAN circuit where you are paying per subnet or where address space is scarce, those extra addresses are wasted. A /30 is the minimum that satisfies the standard IPv4 requirement of having a network address and a broadcast address in addition to host addresses.
The Modern Alternative: /31 (RFC 3021)
RFC 3021 allows /31 subnets on point-to-point links. A /31 has only 2 addresses, and RFC 3021 permits both to be used as host addresses — eliminating the network and broadcast addresses entirely on interfaces where they serve no purpose. See the /31 reference page for the details. Most modern routers support /31, but some legacy equipment does not. The /30 remains more universally compatible.
Allocating /30 Blocks from a Pool
When you have many WAN links to number, allocate /30 subnets from a dedicated management block. For example, assign 10.255.0.0/24 as your point-to-point pool and carve it into 64 non-overlapping /30 subnets: 10.255.0.0/30, 10.255.0.4/30, 10.255.0.8/30, and so on. Each /30 starts at a multiple of 4 because the block size is 4. See the /30 reference for the subnet mask and wildcard details.
Practical Considerations
Label both ends of every point-to-point link in your IP address management system. On a dark day when a router is down and you are troubleshooting by looking at routing tables, knowing that 10.255.0.5 is "Router B, London–Frankfurt link" versus just an anonymous IP address is invaluable. Use the CIDR calculator to verify subnet boundaries when allocating new links.