Subnetting and IPv4 Address Classes
IPv4 TCP/IP Subnet Table
While subnetting might be easy enough to grasp as a concept, it can be a bit involved, and even mind-boggling in part due to the required manipulations of binary numbers. Many people understand the ideas behind subnetting, but find it hard to follow the actual steps required to subnet a network. The table bellow is intended as a quick reference and a fairly complete example of IPv4 subnetting.
| Subnet Mask (Netmask) | Binary | CIDR | Hosts* | Inverse Mask** | Notes |
| 255.255.255.255 | 11111111.11111111.11111111.11111111 | /32 | 1 | 0.0.0.0 | single host mask |
| 255.255.255.254 | 11111111.11111111.11111111.11111110 | /31 | 0 | unusable mask, no host bits | |
| 255.255.255.252 | 11111111.11111111.11111111.11111100 | /30 | 2 | 0.0.0.3 | |
| 255.255.255.248 | 11111111.11111111.11111111.11111000 | /29 | 6 | 0.0.0.7 | |
| 255.255.255.240 | 11111111.11111111.11111111.11110000 | /28 | 14 | 0.0.0.15 | |
| 255.255.255.224 | 11111111.11111111.11111111.11100000 | /27 | 30 | 0.0.0.31 | |
| 255.255.255.192 | 11111111.11111111.11111111.11000000 | /26 | 62 | 0.0.0.63 | |
| 255.255.255.128 | 11111111.11111111.11111111.10000000 | /25 | 126 | 0.0.0.127 | |
| 255.255.255.0 | 11111111.11111111.11111111.00000000 | /24 | 254 | 0.0.0.255 | 1 Class C network |
| 255.255.254.0 | 11111111.11111111.11111110.00000000 | /23 | 510 | 0.0.1.255 | 2 Class C networks |
| 255.255.252.0 | 11111111.11111111.11111100.00000000 | /22 | 1022 | 0.0.3.255 | 4 Class C |
| 255.255.248.0 | 11111111.11111111.11111000.00000000 | /21 | 2046 | 0.0.7.255 | 8 Class C |
| 255.255.240.0 | 11111111.11111111.11110000.00000000 | /20 | 4094 | 0.0.15.255 | 16 Class C |
| 255.255.224.0 | 11111111.11111111.11100000.00000000 | /19 | 8190 | 0.0.31.255 | 32 Class C |
| 255.255.192.0 | 11111111.11111111.11000000.00000000 | /18 | 16382 | 0.0.63.255 | 64 Class C |
| 255.255.128.0 | 11111111.11111111.10000000.00000000 | /17 | 32766 | 0.0.127.255 | 128 Class C |
| 255.255.0.0 | 11111111.11111111.00000000.00000000 | /16 | 65534 | 0.0.255.255 | 1 Class B Network (255 Class C) |
| 255.254.0.0 | 11111111.11111110.00000000.00000000 | /15 | 131070 | 0.1.255.255 | 2 Class B networks |
| 255.252.0.0 | 11111111.11111100.00000000.00000000 | /14 | 262142 | 0.3.255.255 | 4 Class B |
| 255.248.0.0 | 11111111.11111000.00000000.00000000 | /13 | 524286 | 0.7.255.255 | 8 Class B |
| 255.240.0.0 | 11111111.11110000.00000000.00000000 | /12 | 1M | 0.15.255.255 | 16 Class B |
| 255.224.0.0 | 11111111.11100000.00000000.00000000 | /11 | 2M | 0.31.255.255 | 32 Class B |
| 255.192.0.0 | 11111111.11000000.00000000.00000000 | /10 | 4M | 0.63.255.255 | 64 Class B |
| 255.128.0.0 | 11111111.10000000.00000000.00000000 | /9 | 8M | 0.127.255.255 | 128 Class B |
| 255.0.0.0 | 11111111.00000000.00000000.00000000 | /8 | 16M | 0.255.255.255 | 1 Class A Network (255 Class B) |
| 254.0.0.0 | 11111110.00000000.00000000.00000000 | /7 | 32M | 1.255.255.255 | 2 Class A |
| 252.0.0.0 | 11111100.00000000.00000000.00000000 | /6 | 64M | 3.255.255.255 | |
| 248.0.0.0 | 11111000.00000000.00000000.00000000 | /5 | 128M | 7.255.255.255 | |
| 240.0.0.0 | 11110000.00000000.00000000.00000000 | /4 | 256M | 15.255.255.255 | |
| 224.0.0.0 | 11100000.00000000.00000000.00000000 | /3 | 512M | 31.255.255.255 | |
| 192.0.0.0 | 11000000.00000000.00000000.00000000 | /2 | 1024M | 63.255.255.255 | |
| 128.0.0.0 | 10000000.00000000.00000000.00000000 | /1 | 2048M | 127.255.255.255 | |
| 0.0.0.0 | 00000000.00000000.00000000.00000000 | /0 | 4096M | 255.255.255.255 |
* Usable Hosts - (2^N-2) method.
** Inverse mask is calculated by subtracting each subnet mask octet value from 255. The inverse mask is primarily used in Cisco access control lists (ACLs). The rationale behind an inverse mask is that logical ANDing an inverse mask and an IP address gives the host portion of the address, instead of the network / subner portion that a standard subnet mask would. This is of more interest to certain TCP/IP functions.
Notes: Certain potions of the IPv4 address space are specifically allocated by RFCs for special uses, such as loopback (RFC 1643), private networks (RFC 1918), and zeroconf (RFC 3927) usage.
IPv4 Address Classes
Class Address Ranges
Class A - 1.0.0.0 to 126.0.0.0
Class B - 128.0.0.0 to 191.255.0.0
Class C - 192.0.1.0 to 223.255.255.0Class D* - 224.0.0.0 to 239.255.255.255
Class E* - 240.0.0.0 to 255.255.255.255
Class A, Class B, and Class C are the three classes of addresses used on IP networks in common practice. Class D addresses are reserved for multicast. Class E addresses are simply reserved, meaning they should not be used on IP networks (used on a limited basis by some research organizations for experimental purposes).
Reserved Address Ranges
Address ranges below are reserved by IANA for private intranets, and not routable to the Internet. For additional information, see RFC 1918.
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)
Other reserved addresses:
127.0.0.0 is reserved for loopback and IPC on the localhost.
224.0.0.0 - 239.255.255.255 is reserved for multicast addresses.
255.255.255.255 is the limited broadcast address (limited to all other nodes on the LAN)