Subnet Calculator

When designing, deploying, or troubleshooting a network a subnet calculator is a fast way to check the subnet size, wildcard mask, and the address range.

Subnet Results

CIDR Notation:

Wildcard Mask:

Hosts per Subnet:

Subnet Address:

Host Address Range:

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Broadcast Address:

Description

CIDR stands for Classless Inter-Domain Routing. Prior to the creation of CIDR, RFC 1518 and RFC 1519, classful domains where used for network creations. Classful subnets are defined in RFC 791. There were five classes defined, class A, B, C, D, and E. Class A starts at 0.0.0.0 and goes to 127.255.255.255 and is broken into /8 or 255.0.0.0 subnets. Class B starts at 128.0.0.0 and goes to 191.255.255.255 and is broken into /16 or 255.255.0.0. Class C starts at 192.0.0.0 and goes to 223.255.255.255 and is broken into /24 or 255.255.255.0. Class D starts at 224.0.0.0 and goes to 239.255.255.255 and is used for multicast. Class E starts at 240.0.0.0 and goes to 255.255.255.255 and is used for experimental use. When CIDR was introduced it allows larger Class A networks to be broken into smaller components. Likewise smaller subnets like Class C can be combined to form a larger subnet. For example 2 /24 subnets makes 1 /23.

Wildcard Mask is basically an inverted subnet mask. Wildcard masks are useful when configuring routing protocols as the wildcard mask is often used in the route statments to specify which subnet should or should not be routed.

Hosts per Subnet is the number of usable addresses within a scope. The subnet address and broadcast address are not addressable and should not be on a systems network interface card. Since a /31 only has a subnet address and a broadcast address that subnet is very rarely used. However there are some instances where a /31 may be used when writting an Access Control List to permit or restricted from those two addresses rather than having two ACL entries.

Subnet Address is basically the subnet name. This is the subnet for which all computers on that subnet are associated.

Host Address Range is the range of addressable addresses within the subnet. These are the addresses that should be assigned to Layer 3 elements.

Broadcast address is the final address in a subnet and is used by some protocols, such as ARP, to request information from all other systems on the subnet. When a broadcast packet enters the network it will be addressed to the broadcast address and all switches, hubs, or bridges will pass the broadcast message to all systems.