Description of Subnet and subnet masking
Description of Subnet and subnet masking
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The process of dividing a network into two or more networks is called subnetting. A subnet allows the flow of network traffic between hosts to be separated based on a network configuration. By organizing hosts into logical groups, subnetting can improve network security and performance. Like IP addresses, a subnet mask contains four bytes (32 bits) and is often written using the same “dotted-decimal” notation. For example, a very common subnet mask in its binary representation: 11111111 11111111 11111111 00000000 is typically shown in the equivalent, more readable form 255.255.255.0 (Decimal)
IP address is 32 -bits long. In the addressing, one portion of the address indicates a network (net id) and the other portion indicates the host (host id or device) on the network. Applying the subnet mask to an IP address splits the address into two parts, a network address and a host address. For example, consider the IP address 152.168.4.254. Assuming this is part of a Class B network, the first two numbers (152.168) represent the Class B network address, and the second two numbers (4.257) identify a particular host on this network. Subnetting enables the network administrator to further divide the host part of the address into two or more subnets (sub networks). In this case, a part of the host address is reserved to identify the particular subnet. By subnetting administrator can assign more IP addresses to different host or devices. Subnetting an IP network is to separate a big network into smaller multiple networks for reorganization and security purposes, that means applying a subnet mask to an IP address separates network address from host address. All nodes (hosts) in a subnetwork see all packets transmitted by any node in a network. IP addressing is designed with two-level of hierarchy. To reach a host on the internet, we must first reach to the network using the first portion of the address (net id). Then we must reach to the host itself using the second portion (host id). It means it goes from net id to the host id. Sometimes this two-level of hierarchy is not suitable to the organization, so at the point network needs to be divided into several smaller networks. The further division of a network into smaller networks called sub network or in short subnet. For example, University has many departments. The university itself considered as a network and has one network address, but its departments have several sub network addresses. Each sub network is identified by its sub network address. When we divide a network into several subnets we have three-level hierarchy.
[Network Sub network Host]
Subnet masking: When a router receives a packet with a destination address, it needs to route the packet. The routing is based on the network address and sub network address. The router outside the organization (network) routes the packet based on the network address. The router inside the organization routes the packet based on the sub network address. How can router find the network address or sub network address? A network administrator knows the network address and sub network address but router does not. Router uses the masking process. Masking is a process that extracts the address of the network from an IP address. Masking can be done whether we have sub netting or not. If we have not sub netted the network, masking extracts the network address from an IP address. If we have sub netted, masking extracts the sub network address from an IP address.
Examples of commonly used net masks for classed networks are 8-bits (Class A), 16-bits (Class B) and 24-bits (Class C).Default subnet masks:
· Class A – 255.0.0.0 – 11111111.00000000.00000000.00000000
· Class B – 255.255.0.0 – 11111111.11111111.00000000.00000000
· Class C – 255.255.255.0 – 11111111.11111111.11111111.00000000
Consider another example in below figure that shows network 139.12.0.0 before subnetting. The class B network of 139.12.0.0 can have up to 65,534 nodes. This is far too many nodes, and in fact the current network is becoming saturated with broadcast traffic. The subnetting of network 139.12.0.0 should be done in such a way so that it does not impact nor require the reconfiguration of the rest of the IP internetwork.
[Fig (a): Before subnetting]
Below figure shows network 139.12.0.0 after subnetting. Network 139.12.0.0 is subnetted by utilizing the first 8 host bits (the third octet) for the new subnetted network ID. When 139.12.0.0 is subnetted, as shown in Figure, separate networks with their own subnetted network IDs (139.12.1.0, 139.12.2.0, 139.12.3.0) are created. The router is aware of the separate subnetted networks IDs and routes IP packets to the appropriate subnet. Note that the rest of the IP internetwork still regards all the nodes on the three subnets as being on network 139.12.0.0. The other routers in the IP internetwork are unaware of the subnetting being done on network 139.12.0.0 and therefore require no reconfiguration. 
[Fig (a): Before subnetting]
But how does the router know who is subdividing network 139.12.0.0? How the network is being subdivided and which subnets are available on which router interfaces? To give the IP nodes this new level of awareness, they must be told exactly how to separate the new subnetted network ID. A subnet mask is used to tell an IP node how to extract a subnetted network ID.