SubnetPad

/31 Subnet Calculator

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A /31 block contains 2 addresses — both usable for point-to-point links (RFC 3021). Unlike /30, there is no network or broadcast address, saving 2 IPs. It is now preferred over /30 for router-to-router links.

/1
/32

/31 = mask 255.255.255.254 • 2 usable addresses

Results for 192.168.1.0/31

Private (RFC 1918)Class C
Network addressThe first address — identifies the subnet itself
Broadcast addressThe last address — used to reach every device at once
Subnet maskSame as /31 in CIDR notation
First usable hostThe first IP you can assign to a device
Last usable hostThe last IP you can assign to a device
Usable hosts2 total addresses in this subnet
More detailsNeighboring subnets, wildcard mask, hex, reverse DNS, and binary breakdown

Neighboring /31 subnets

Wildcard mask
Used in Cisco ACLs and OSPF
CIDR notation
IP in hexadecimal
IP as 32-bit integer
Reverse DNS (PTR)
Host bits / subnet bits

Binary breakdown

IP address11000000.10101000.00000001.00000000
Subnet mask11111111.11111111.11111111.11111110
Network11000000.10101000.00000001.00000000
Broadcast11000000.10101000.00000001.00000001

Quick facts for /31

/31 reference guideBit split, overview, sizing tables, notable networks, and FAQ

Binary bit split

Network bits (31)Host bits (1)
Network bits
31
Host bits
1
Subnet mask
255.255.255.254
Wildcard mask
0.0.0.1
Total addresses
2
Usable hosts
2
Hosts formula
2^1 (RFC 3021)
/24 relationship
128 fit per /24

Overview

A /31 block contains 2 addresses — both usable for point-to-point links (RFC 3021). Unlike /30, there is no network or broadcast address, saving 2 IPs. It is now preferred over /30 for router-to-router links.

Common use cases

  • RFC 3021 point-to-point router links
  • Cisco, Juniper, and bird router interconnects
  • Efficient WAN link addressing in ISP networks
  • Modern replacement for /30 on p2p links

Subnet sizing reference

Divide /31 into…

PrefixSubnetsHosts each
/3221

/31 fits inside…

SupernetHosts/31s inside
/3022
/2964
/28148
/273016

Notable /31 networks

  • 10.0.0.0/312 usable hosts: .0 and .1
  • 192.168.1.0/31RFC 3021 p2p example

Frequently asked questions

Common questions about /31 networks, subnet masks, and use cases.

All CIDR prefix sizes

/0/1/2/3/4/5/6/7/8/9/10/11/12/13/14/15/16/17/18/19/20/21/22/23/24/25/26/27/28/29/30/31/32