In modern networking, routers frequently connect directly to other routers across long-distance fiber runs, leased telephone lines, or microwave links. On these dedicated point-to-point cables, there are only two interfaces in existence: the router interface at the headquarters building and the router interface at the branch office.
If you assign a standard /24 subnet to this link, you dedicate 256 IP addresses to a cable where only two devices will ever live. Two hundred and fifty-four addresses sit completely unused, locked inside a routing entry where nobody else can touch them. In an era where IPv4 addresses are scarce and expensive, this kind of waste is unacceptable.
The Math of the /30 Subnet Mask
To provide exactly two usable host IP addresses, we borrow thirty bits for the network portion, leaving only two bits for host addressing. The binary mask looks like this: 11111111.11111111.11111111.11111100. In dotted decimal notation, this corresponds to 255.255.255.252.
With two host bits, the total number of combinations is 2^2 = 4 addresses. After subtracting the mandatory network address and broadcast address, exactly two usable host addresses remain: 4 - 2 = 2. One address goes to Router 1, and the other address goes to Router 2. Zero addresses are wasted.
Because the block size is 4 (256 - 252 = 4), point-to-point subnets advance in compact increments of four across the entire address space. Network engineers memorize these increments: 0, 4, 8, 12, 16, 20, 24, 28, and so forth. Every multiple of 4 represents a new independent point-to-point interconnect.
| Subnet Block | Network ID | Router 1 IP | Router 2 IP | Broadcast ID |
|---|---|---|---|---|
| 10.0.0.0/30 | 10.0.0.0 | 10.0.0.1 | 10.0.0.2 | 10.0.0.3 |
| 10.0.0.4/30 | 10.0.0.4 | 10.0.0.5 | 10.0.0.6 | 10.0.0.7 |
| 10.0.0.8/30 | 10.0.0.8 | 10.0.0.9 | 10.0.0.10 | 10.0.0.11 |
| 10.0.0.12/30 | 10.0.0.12 | 10.0.0.13 | 10.0.0.14 | 10.0.0.15 |
Why /30 Remains the Enterprise Standard
Some modern networking standards propose using /31 subnet masks (RFC 3021) for point-to-point links, which eliminates the network and broadcast address reservations entirely. However, many legacy routers, older firewall appliances, and industrial controllers do not support /31 addressing.
Because of this, /30 remains the universal, fail-safe industry standard across telecom carriers and enterprise WANs worldwide. Every router manufactured over the past thirty years recognizes a /30 subnet instantly, making it the most reliable choice for multi-vendor network connections.
Furthermore, network monitoring systems often rely on the distinct broadcast address of a /30 link to identify boundary limits in automated network topology discovery maps. Sticking with /30 ensures your monitoring dashboards render link states accurately.
Configuring /30 Interfaces on Real Equipment
When entering IP settings on router interfaces, you specify the IP address followed by the mask 255.255.255.252. On Cisco IOS command line, this is written as ip address 10.0.0.1 255.255.255.252. On Mikrotik RouterOS, it is expressed cleanly in CIDR format: /ip address add address=10.0.0.1/30 interface=ether1.
Because the network size is so tight, entering a typo—such as assigning 10.0.0.3 to an interface—causes immediate configuration errors. The router will warn you that 10.0.0.3 is a reserved broadcast address, preventing accidental broadcast storms.
Notice also that point-to-point links do not require default gateway entries on their specific interfaces. Because the interface knows it is directly wired to the peer router, the operating system can route packets across the wire using interface direct-delivery mechanisms.
Verifying Point-to-Point Link Operation
Once both router interfaces are configured with their respective /30 IPs, the link status transitions to UP/UP. Because no intermediate switches or hubs exist between them, round-trip ping latency across a point-to-point link is virtually instantaneous (under 1 millisecond).
In our lab exercise, you will connect Headquarters Router RTR-HQ to Branch Router RTR-BR using a /30 WAN link, verify interface routing status, and test bidirectional communication across the dedicated point-to-point interconnect.
You will also inspect the routing table to see how a /30 route appears. Modern routers create both a /30 connected route for the subnet and a /32 local route for the router own interface address, providing precise packet processing at the hardware layer.