How many usable hosts does a /31 subnet have?

A /31 subnet has 2 IP addresses. Per RFC 3021, both addresses can be used as host addresses on point-to-point links — there is no network address or broadcast address in a /31. Traditional subnetting rules would give 0 usable hosts, but RFC 3021 defines a special case for exactly this purpose.

Can you use a /31 in AWS VPC?

No. AWS VPC does not support /31 subnets. The minimum subnet size in AWS VPC is /28, which provides 16 total IP addresses and 11 usable after AWS reserves 5. The /31 is only used in traditional networking hardware for point-to-point WAN links.

When should I use a /31 instead of a /30?

Use /31 (RFC 3021) instead of /30 when you want to conserve IP address space on point-to-point links. A /30 wastes 2 addresses (network + broadcast) to connect only 2 hosts. A /31 uses exactly 2 addresses for the same 2 hosts, saving half the address space. Modern routers (Cisco IOS 12.2+, Juniper, Arista) all support /31.

What is a /31 Subnet? RFC 3021 Point-to-Point Links Explained

What is a /31 Subnet?

Last reviewed: May 2026

A /31 subnet contains exactly 2 IP addresses. Unlike any other subnet size, the /31 is governed by a special rule: RFC 3021 defines that both addresses in a /31 can be used as host addresses on point-to-point links, with no network address or broadcast address reserved. This makes /31 the most IP-efficient way to number a router-to-router connection.

Calculate any /31 subnet instantly →

Quick Facts: 192.168.1.0/31 Subnet mask: 255.255.255.254 · Wildcard: 0.0.0.1 · Address 0: 192.168.1.0 (router A) · Address 1: 192.168.1.1 (router B) · Total IPs: 2 · Usable hosts (RFC 3021): 2

RFC 3021 — Using 31-Bit Prefixes on IPv4 Point-to-Point Links

Published in December 2000, RFC 3021 defines that when a subnet mask of /31 is applied to a point-to-point link, the two addresses in the block are treated as host addresses — the concept of network address and broadcast address does not apply. This is supported by all modern routing platforms: Cisco IOS 12.2+, Juniper Junos, Arista EOS, Nokia SR OS, and others.

Full Breakdown

Property	Value
Prefix length	/31
Subnet mask	255.255.255.254
Wildcard mask	0.0.0.1
Total addresses	2
Usable hosts (RFC 3021)	2 — both addresses assignable on point-to-point links
Usable hosts (traditional)	0 — traditional math: 2¹ − 2 = 0
Network address	Not applicable (per RFC 3021)
Broadcast address	Not applicable (per RFC 3021)
Binary mask	11111111.11111111.11111111.11111110
AWS VPC support	Not supported — minimum is /28

Property

Value

Prefix length

/31

Subnet mask

255.255.255.254

Wildcard mask

0.0.0.1

Total addresses

Usable hosts (RFC 3021)

2 — both addresses assignable on point-to-point links

Usable hosts (traditional)

0 — traditional math: 2¹ − 2 = 0

Network address

Not applicable (per RFC 3021)

Broadcast address

Not applicable (per RFC 3021)

Binary mask

11111111.11111111.11111111.11111110

AWS VPC support

Not supported — minimum is /28

/31 vs /30 — Saving Half the Address Space

A /30 has 4 addresses: 1 network + 2 hosts + 1 broadcast. Using /30 for every WAN link wastes 2 out of every 4 addresses. A /31 uses just 2 addresses for the same 2 hosts — 50% more efficient. For an ISP with thousands of point-to-point links, this saves a significant portion of the IPv4 address pool.

/31 vs /30 Side-by-Side

Property	/31 (RFC 3021)	/30 (traditional P2P)
Total addresses	2	4
Usable hosts	2	2
Wasted addresses	0	2 (network + broadcast)
AWS VPC support	No	No (below /28 minimum)
Modern router support	Yes (IOS 12.2+, Junos, Arista)	Yes (universal)
BGP session support	Yes — preferred by many ISPs	Yes

Property

/31 (RFC 3021)

/30 (traditional P2P)

Total addresses

Usable hosts

Wasted addresses

2 (network + broadcast)

AWS VPC support

No (below /28 minimum)

Modern router support

Yes (IOS 12.2+, Junos, Arista)

Yes (universal)

BGP session support

Yes — preferred by many ISPs

Yes

Common Use Cases for /31 Subnets

ISP backbone links — every router-to-router WAN link; /31 is the standard in modern ISP networks

BGP peering sessions — numbering the physical interface of an eBGP peering session between routers

Data centre interconnects — links between spine and leaf switches where exactly two devices are connected

VPN tunnel endpoints — assigning interface IPs to a GRE or IPsec tunnel with two endpoints

Lab point-to-point links — conserving IP space in lab environments with many router-to-router connections

Device	IP Address	Role
Router A (your ASN)	203.0.113.0/31	eBGP peer, interface address
Router B (peer ASN)	203.0.113.1/31	eBGP peer, interface address

Device

IP Address

Role

Router A (your ASN)

203.0.113.0/31

eBGP peer, interface address

Router B (peer ASN)

203.0.113.1/31

eBGP peer, interface address

Comparison: /31 and Nearby Small Prefixes

CIDR	Subnet Mask	Total IPs	Usable Hosts	AWS Usable	Typical Use
/28	255.255.255.240	16	14	11	AWS minimum; small managed subnet
/29	255.255.255.248	8	6	3	Small server cluster; mgmt VLAN
/30	255.255.255.252	4	2	—	Traditional point-to-point link
/31	255.255.255.254	2	2 (RFC 3021)	—	Efficient P2P link — ISP/backbone
/32	255.255.255.255	1	1 host route	—	Loopback, host route, anycast

CIDR

Subnet Mask

Total IPs

Usable Hosts

AWS Usable

Typical Use

/28

255.255.255.240

AWS minimum; small managed subnet

/29

255.255.255.248

Small server cluster; mgmt VLAN

/30

255.255.255.252

—

Traditional point-to-point link

/31

255.255.255.254

2 (RFC 3021)

—

Efficient P2P link — ISP/backbone

/32

255.255.255.255

1 host route

—

Loopback, host route, anycast

Calculate /31 point-to-point subnets →