Subnetting

By Neil Lynch

Subnet Masking
Subnet masks tell the computer or router which part of an IP address is the network portion and which part is the host portion.
Subnet masking is a manipulation of sorts in the host’s octets of the IP classes, Class A, Class B, and Class C. It is sometimes done to control the flow of traffic in terms of hosts per segment, etc. According to Cannon, et al., Manipulating the mask via subnetting is a big improvement over fixed-length mask because it allows a single major network number to be subdivided into smaller subnetworks (102). ”Whenever you subnet
a network address you lose some of the host addresses that you could have had without subnetting” (Cannon 96). By masking a third octet in a Class B, IP address, you reduce the number of addresses significantly.

Network administrators can use up to 14 bits to subnet a Class B address and six bits of Class C host octet.

The best way to learn to subnet a network is to use a Class C address; it allows you to subnet the last octet. For example, let’s look at subnetting with the current lab project as illustrated, (Cannon, et al., 464)  

In this network there were segments with various hosts and interfaces. 

The Network IP was 192.3.2.0; the  interfaces were F0/0, F0/1, and S0/0 with 51, 4, and 2 hosts        

respectively.

 

Since the network was Class C, I took the IP address 192.3.2.0 and masked its host by 2bits (2^y =4), where y = 2. This gave me four (4) subnets with 6 host portions remaining.

 

My right-most bit was 2^6 or 64 in decimal which is what I used as my incremental value for the four subnets: 192.3.2.0/26, 192.3.2.64/26, 192.3.2.128/26, 192.3.2.192/26.

Each address now had 62 hosts which accommodated Net 1 and Net 2. 

Net 3, on the other hand, the third       subnet…must accommodate (2)hosts. With the host portion of 6 remaining, and only 2 bits are required for this host, I move the mask 4 bits to the right, making it /30. With y=4, we get 2^4=16 more subnets, each with 2 host IP addresses:192.3.2.64/30, 192.3.2.68/30 through 192.3.2.124/30

 

 

 

 

 

                                                                             References

Cannon, Kelly. Kelly Caudle, Anthony Chiarella. CCNA Guide to Cisco Networking 4/e. Boston: 2009, Print.