CIDR /31Private (RFC 1918)
/31 Subnet Mask (255.255.255.254) - Usable Hosts & CIDR | ipcalc.net
Everything about /31 subnets: Subnet mask 255.255.255.254, wildcard mask 0.0.0.1, 2 usable hosts, binary breakdown, and Cisco configuration templates.
Enter any notation: 192.168.1.1/24, 10.0.0.1 255.255.255.0, or plain IP
Subnet Mask Prefix: /31Netmask: 255.255.255.254
Presets:
32-Bit Subnet Boundary Visualizer
Click any bit position to adjust the CIDR prefix boundary
Network (31 bits)
Host (1 bits)
Octet 1192Mask: 255
11111111
Octet 2168Mask: 255
11111111
Octet 31Mask: 255
11111111
Octet 40Mask: 254
11111110
Usable Hosts
2
Total IPs: 2
Network Address
192.168.1.0
Prefix: /31
Usable Host Range
192.168.1.0
192.168.1.1
Broadcast Address
None (RFC 3021 Point-to-Point)
RFC 3021 Point-to-Point
Detailed Subnet Breakdown
Private (RFC 1918)Subnet Mask
255.255.255.254
Wildcard Mask (Cisco ACL)
0.0.0.1
Hexadecimal IP / Mask
0xC0A80100 / 0xFFFFFFFE
Integer / Decimal IP
3232235776
IP Class & Routing
Class C • Classless Inter-Domain Routing (CIDR)
Reverse DNS (in-addr.arpa)
0.1.168.192.in-addr.arpa
Cisco IOS Command Templates
ip address 192.168.1.0 255.255.255.254
access-list 1 permit 192.168.1.0 0.0.0.1
Technical Subnet Specifications
Prefix Length/31
Dotted Decimal Subnet Mask255.255.255.254
Wildcard Mask (Inverse)0.0.0.1
Total Available IP Addresses2
Usable Host Capacity2 hosts
Network Bits vs Host Bits31 Network / 1 Host
Binary Subnet Mask11111111.11111111.11111111.11111110
Cisco IOS Configuration
Copy-paste templates for Cisco router and switch interface setup:
! Configure Interface
interface GigabitEthernet0/0/1
ip address 192.168.1.0 255.255.255.254
no shutdown
! Access-Control List
access-list 10 permit 192.168.1.0 0.0.0.1
Tip: Wildcard mask is computed by inverting the subnet mask (
~mask).Typical Network Use Cases for /31
A /31 network block provides 2 total IP addresses with 2 usable host interfaces.
Designated Role: Point-to-Point Link (RFC 3021).
RFC 3021 Note: /31 subnets are specifically designed for point-to-point router links. Because there are exactly two interfaces on the wire, there is no need for broadcast or network address reservation.
Frequently Asked Questions: /31 (255.255.255.254)
Authoritative answers to common /31 subnetting, host capacity, and networking questions.
Q1How many usable host IP addresses are in a /31 subnet?
A /31 subnet (netmask 255.255.255.254) provides exactly 2 usable IP addresses under RFC 3021. Because /31 subnets are exclusively used for point-to-point router links between two devices, neither a dedicated network address nor a broadcast address is reserved, preventing address waste on routing transit links.
Q2What is the subnet mask and wildcard mask for /31?
The dotted-decimal subnet mask for a /31 prefix is 255.255.255.254. The corresponding wildcard mask (used in Cisco ACLs and OSPF network statements) is 0.0.0.1. In binary, the subnet mask consists of 31 network bits (1s) followed by 1 host bit (0s).
Q3What is a /31 subnet typically used for in network design?
A /31 subnet is typically deployed for point-to-point link (rfc 3021). In enterprise and cloud environments (such as AWS VPCs or Azure VNets), /31 allocations provide the optimal balance between host capacity and address conservation.
Q4What does /31 mean in CIDR notation?
Classless Inter-Domain Routing (CIDR) notation expresses the subnet mask as a forward slash followed by the prefix length. A /31 prefix indicates that the first 31 bits of the 32-bit IPv4 address are locked for network identification, leaving the remaining 1 bit available for addressing host interfaces.