How is the number of usable hosts calculated in a subnet?

The formula for usable IPv4 hosts is 2^(32 - CIDR) - 2. We subtract 2 because the first address is reserved for the Network ID and the last address is reserved as the Subnet Broadcast address.

What is a Wildcard Mask in networking?

A wildcard mask is the inverted binary representation of a subnet mask (255.255.255.255 minus Subnet Mask). It is commonly used in Cisco IOS Access Control Lists (ACLs) and OSPF/EIGRP routing protocol configurations.

What are RFC 1918 Private IP Ranges?

RFC 1918 defines three non-routable private IPv4 blocks for local networks: 10.0.0.0/8 (10.0.0.0 - 10.255.255.255), 172.16.0.0/12 (172.16.0.0 - 172.31.255.255), and 192.168.0.0/16 (192.168.0.0 - 192.168.255.255).

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CIDR Subnet Calculator

Instantly compute network addresses, broadcast IDs, usable host ranges, and binary masks for any IPv4 CIDR prefix.

IPv4 Address

Subnet Mask / CIDR

Presets:

Network ID Address

192.168.1.0

Broadcast Address

192.168.1.255

Usable Host IP Range

192.168.1.1 - 192.168.1.254

Total Usable Hosts

254

2^(32-n) - 2

Subnet Mask

255.255.255.0

Cisco Wildcard Mask (ACLs)

0.0.0.255

IP Class & Routing Scope

Class C RFC 1918 Private

Hexadecimal Mask

0xFFFFFF00

32-Bit Binary Structure Network Bits (Blue) vs Host Bits (Gray)

IP Address: 11000000.10101000.00000001.00000001

Subnet Mask: 11111111.11111111.11111111.00000000

Network ID: 11000000.10101000.00000001.00000000

IPv4 CIDR Prefix & Subnet Mask Reference Table

/8 to /32

CIDR Prefix	Subnet Mask	Usable Hosts	Total IPs	Standard Class
/32	255.255.255.255	1	1	Single Host / Loopback
/30	255.255.255.252	2	4	Point-to-Point Link
/28	255.255.255.240	14	16	Small Branch / VLAN
/24	255.255.255.0	254	256	Class C (Home Routers)
/20	255.255.240.0	4,094	4,096	Medium Enterprise
/16	255.255.0.0	65,534	65,536	Class B
/8	255.0.0.0	16,777,214	16,777,216	Class A

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Subnetting Frequently Asked Questions

How is the number of usable hosts calculated?

The formula for usable IPv4 hosts is 2^(32 - CIDR) - 2. We subtract 2 because the first address in any block is reserved for the Network ID and the last address is reserved for broadcast traffic.

What is a Wildcard Mask used for?

A wildcard mask is the inverted binary representation of a subnet mask (255.255.255.255 minus Subnet Mask). It is extensively used in Cisco IOS Access Control Lists (ACLs) and OSPF routing configurations to match ranges of IP addresses.

What are the RFC 1918 Private IP Ranges?

RFC 1918 allocates three specific IPv4 address blocks for private local networks: 10.0.0.0/8 (10.0.0.0 to 10.255.255.255), 172.16.0.0/12 (172.16.0.0 to 172.31.255.255), and 192.168.0.0/16 (192.168.0.0 to 192.168.255.255).