Sonora Communications, Inc.

Use our handy On-Line IP Subnet Calculator!

Most all data networks today use IP.  IP stands for Internet Protocol.

An IP address as you probably know it is a 'dotted decimal' representation of a binary (address) number:

192.168.1.1 = 11000000.10101000.00000001. 00000001

An IP address tells you what network you are on and what your host ID (address) is.  The subnet mask tells you (the host) what part of the IP address specifies the network and the part left over tells you the host ID (address). 

The netmask specifies the size of the subnet, i.e how many hosts it may contain.  In the above example, a /24 or 255.255.255.0 subnet mask tells us that all the bits up to the last octet specify the network and the last octet specifies the host.

Traditional IP networks come in three classifications:

• Class A
  • netmask 255.0.0.0
  • netmask /8 using CIDR notation
  • 16777214 possible hosts per net

• Class B
  • netmask 255.255.0.0
  • netmask /16 using CIDR notation
  • 65534 possible hosts per net

• Class C
  • netmask 255.255.255.0
  • netmask /24 using CIDR notation
  • 254 possible hosts per net

Since the three traditional subnet sizes don't provide much flexibility, Classless InterDomain Routing or CIDR subnetting comes to the rescue.  CIDR subnetting allows us much more granular control.  For example, it is common for us to assign small blocks of public IP addresses to our customers that need them.  We often assign /29 or /30 subnets:

Other resources on the web:

• http://www.ralphb.net/IPSubnet/index.html
• http://www.cisco.com/warp/public/701/3.html
• http://www.pantz.org/networking/tcpip/subnetchart.shtml
• http://articles.techrepublic.com.com/5100-1035-6089187.html