Subnet Calculator

A subnet mask is a 32-bit number that divides an IP address into network and host portions:

• The network portion identifies which network the device belongs to
• The host portion identifies the specific device on that network
• Subnet masks use consecutive 1s followed by consecutive 0s in binary
• Common masks include 255.255.255.0 (/24), 255.255.0.0 (/16), and 255.0.0.0 (/8)

For example, with a /24 subnet mask, the first 24 bits identify the network and the last 8 bits identify hosts.

CIDR (Classless Inter-Domain Routing) notation is a compact way to specify IP addresses and their associated subnet masks:

• Written as IP address followed by a slash and the number of network bits (e.g., 192.168.1.0/24)
• The number after the slash indicates how many bits are used for the network portion
• /24 means the first 24 bits are the network part (255.255.255.0)
• /16 means the first 16 bits are the network part (255.255.0.0)
• Larger CIDR numbers mean smaller networks with fewer hosts

CIDR replaced the older classful addressing system and allows more flexible allocation of IP addresses.

IP address classes are a legacy classification system based on the first octet of an IP address:

• Class A (1-126): Large networks with up to 16 million hosts. Default mask: /8
• Class B (128-191): Medium networks with up to 65,534 hosts. Default mask: /16
• Class C (192-223): Small networks with up to 254 hosts. Default mask: /24
• Class D (224-239): Reserved for multicast groups
• Class E (240-255): Reserved for experimental use

Note: 127.x.x.x is reserved for loopback addresses (localhost).

Network and broadcast addresses are special addresses in every subnet that cannot be assigned to hosts:

• Network Address: The first address in a subnet, with all host bits set to 0. It identifies the subnet itself.
• Broadcast Address: The last address in a subnet, with all host bits set to 1. Messages sent here reach all hosts in the subnet.
• These two addresses reduce usable hosts by 2 in each subnet
• For 192.168.1.0/24: network is 192.168.1.0, broadcast is 192.168.1.255

Usable host addresses are everything between the network and broadcast addresses.

A wildcard mask is the inverse of a subnet mask, commonly used in access control lists (ACLs) and routing protocols:

• Calculated by subtracting each subnet mask octet from 255
• For subnet mask 255.255.255.0, wildcard is 0.0.0.255
• 0 bits in wildcard mean "must match exactly"
• 1 bits in wildcard mean "can be any value"
• Used extensively in Cisco IOS for ACLs and OSPF configuration

Think of it as: subnet mask defines the network, wildcard mask defines what can vary.