/30 or /31 for point-to-point links: which to use

Point-to-point links between routers, firewalls, and other network devices have unique addressing requirements. Unlike LANs where you need room for many hosts, a point-to-point link connects exactly two devices. The choice between /30 and /31 subnets affects address efficiency, compatibility, and operational practices. Understanding when to use each is essential for modern network design and certification exam readiness.

The /30 subnet: traditional approach

A /30 subnet provides 4 addresses divided by a /30 prefix length. The breakdown is: network address, first usable host, second usable host, and broadcast address. For example, 192.168.1.0 divided by 30 gives you 192.168.1.0 (network), 192.168.1.1 (router A), 192.168.1.2 (router B), and 192.168.1.3 (broadcast).

192.168.1.0/30
  Network:   192.168.1.0
  Host 1:    192.168.1.1
  Host 2:    192.168.1.2
  Broadcast: 192.168.1.3

The /30 has been the standard for decades. It works on all platforms, requires no special configuration, and is universally understood. Many legacy systems, older IOS versions, and vendor implementations expect this model. If you are working in a mixed-vendor environment or supporting older equipment, /30 is the safe choice.

The /31 subnet: RFC 3021 and address efficiency

RFC 3021 (published in 2000) formally permits /31 subnets on point-to-point links. A /31 provides only 2 addresses: one for each end of the link. There is no network address, no broadcast address, and no wasted space. For 192.168.1.0 divided by 31, you get 192.168.1.0 and 192.168.1.1 only.

192.168.1.0/31
  Host 1: 192.168.1.0
  Host 2: 192.168.1.1

(No network or broadcast address)

In modern networks, /31 is the preferred choice. It cuts address waste in half and scales efficiently across large numbers of point-to-point links. Modern routing platforms—including Cisco IOS XE, Juniper Junos, Arista EOS, and others—support RFC 3021 natively. At scale, the address savings become significant: a network with 1,000 point-to-point links saves 1,000 addresses by using /31 instead of /30.

When to use each

• →Use /30 if: you support legacy equipment, work with older IOS versions (pre-12.4), operate in multi-vendor environments where /31 support is uncertain, or follow organizational policy requiring /30.
• →Use /31 if: you run modern platforms, design greenfield networks, need to conserve address space, build large-scale fabrics (data center, service provider), or are studying for current certifications (CCNA, NSE, JNCIA).
• →Mixed environments: document your choice clearly. Mixing /30 and /31 across different links is acceptable; inconsistency is not a technical problem, only an operational one.

Practical configuration examples

! Cisco IOS - /31 point-to-point link
interface GigabitEthernet0/0/0
 ip address 10.0.1.0 255.255.255.254
 no shutdown

! Juniper Junos - /31 point-to-point link
set interfaces ge-0/0/0 unit 0 family inet address 10.0.1.0/31

! Arista EOS - /31 point-to-point link
interface Ethernet1
 ip address 10.0.1.0/31

Configuration syntax is identical whether you use /30 or /31; the difference is purely in the prefix length and the second address. Routing protocols (BGP, OSPF, ISIS) work transparently on both. No special commands or flags are required on modern platforms.

Key takeaways

• →/30 is traditional, universally compatible, and safe for mixed or legacy environments.
• →/31 (RFC 3021) is efficient, modern, and the industry standard for new deployments.
• →Both are functionally identical in modern networks; the choice is operational, not technical.
• →Document your standard and apply it consistently across your organization.
• →For certification exams, know both models and understand when each applies.

The right choice depends on your environment, platform support, and organizational standards. In greenfield designs and modern networks, /31 is the default. In legacy or mixed environments, /31 may not be an option. Either way, understanding the trade-offs ensures you design efficient, maintainable point-to-point addressing schemes.