IP Addressing Facts in Network Infrastructure

The following table lists the default IP addressing classes and masks:

Class	Address Range	Default Mask
A	1.0.0.0 to 126.255.255.255	255.0.0.0
B	128.0.0.0 to 191.255.255.255	255.255.0.0
C	192.0.0.0 to 223.255.255.255	255.255.255.0
D	224.0.0.0 to 239.255.255.255	(multicast addresses)
E	240.0.0.0 to 255.255.255.255	(experimental addresses)

You should also know the following address ranges that are reserved for private addresses. Use these addresses on a private network that is connected to the Internet through a network address translation (NAT) router.

• 10.0.0.0 to 10.255.255.255
• 172.16.0.0 to 172.31.255.255
• 192.168.0.0 to 192.168.255.255

Keep in mind the following facts about IP addresses:

• The first address in a range on the subnet is the subnet address. Typically, this address is not assigned to hosts.
• The last address in a range on the subnet is the broadcast address. Typically, this address is not assigned to hosts.

Use the table as a shortcut guide to subnetting. Tip: Look for patterns in the table so you can easily reproduce the table at any time.

Masked Bits	Mask Value	Number of Subnets*	Number of Hosts/Subnet**	Approximate*** Number of Hosts/Subnet
/20	255.255.240.0	N/A	4096	4000
/21	255.255.248.0	N/A	2048	2000
/22	255.255.252.0	N/A	1024	1000
/23	255.255.254.0	N/A	512	500
/24	255.255.255.0	1	256	250
/25	255.255.255.128	2	128	125
/26	255.255.255.192	4	64	60
/27	255.255.255.224	8	32	30
/28	255.255.255.240	16	16	15

*The number of subnets value is important when you need to subdivide a Class C address range into multiple subnets.
**Remember to subtract two from these numbers to arrive at the total number of addresses that can be assigned to hosts (subtract one for the subnet address and one for the broadcast address.
***Use this number as a quick estimate of the number of possible hosts.