DIY Subnetting

IPs allow your hosts to connect to a network.

Reasons for subnetting Internet Protocol (IP) blocks include smaller network loads, better performance and simpler management. Subnetting networks can be easily done with a calculator or by your provider. Subnetting is one skill that you could never develop on your own but if you do, it will open a lifetime of opportunities.

Instructions

1. Identify the number of networks that are needed. This network could be just one for your whole business or multiple networks. The advantage of using multiple networks allows you to separate traffic such as one customer from another or even customer traffic from the management traffic for your network. Once you have identified the networks that you will be assigning IPs, identify the amount of network IPs that you need.

2. Identify the number of devices that will need an IP. Some devices have more than one network connection, so be sure to include multiple interfaces in your count for hosts if they each need their own IP. Count each host and note if any special requirements are needed that would cause the IPs to be on a different network then other hosts surrounding the device. (You can pick a host address off of a different network IP and then traffic can be limited between the two different networks.)

3. Decide if you will need private addresses or IPs that can be routed over the Internet. Private IPs can be allocated without any preauthorization from additional sources. If you need IPs that can be routed over the Internet, contact your provider or other resource to be assigned a large block of IPs. From the larger block you will make the smaller allocation. A typical block used for private addressing is 192.168.0.0/23. This address contains the IP addresses 192.168.0.0 to 192.168.1.255.

4. Allocate networks within these blocks. Find the network portion of the address. Each address is base2, which means that each position is worth 2 or has two possible values. In this case, a 1 or a 0. The networks in this case can be split into two /24 addresses -- 192.168.0.0 and 192.168.1.0. Each number between the dots represents an eight bit binary number. The two networks in this case would look something similar to 11000000.10101000.00000000.00000000 or 11000000.10101000.00000001.00000000. Count from left to right to find the place where the network is divided and where the network address is -- verses the host address. All of the numbers to the left belong to the network and the numbers on the right belong to the host portion of the address.

5. Setting all positions to 1 or 11111111 equals 255. Never use the first or the last address. The first address is the network address and the last is a broadcast address, which is used like a bull horn to summons all IPs on that network if a packet gets lost and can't find its way. A packet that is addressed to that final broadcast IP will go to every host until the one that needed the packet responds with an acknowledgement to the sender. Any other IPs 192.168.0.1 to 192.168.0.254 can then be assigned to the hosts on your network. Repeat this process as many times as necessary to allocate IPs to all of the devices on your network.

Tips & Warnings

• Additional IPs can be assigned or added to, once the addresses are in use.

• Do not make a network change during the day or without a change process in place as this may affect network connectivity for the customers.

References

• "Cisco Routers for the Small Business: A Practical Guide for IT Professionals"; Jason C. Neumann; 2009
• "CCENT Certification All-In-One For Dummies"; Glen E. Clarke; 2011
• "CompTIA Network+ Deluxe Study Guide: (Exam N10-004)"; Todd Lammle; 2009
• "The TCP/IP Guide"; Charles M. Kozierok; 2005
• "Network Routing Basics: Understanding IP Routing in Cisco Systems; John Wiley & Sons; 2006.

Resources

• Cisco: Host and Subnet Quantities
• Cisco: IP Addressing and Subnetting for New Users
• Cisco: Configuring IPv4 Addresses