NA Lesson - 11

Understanding Subnetting

Session Objectives

• Manipulate subnet  masks to configure subnets for a variety of network restrictions and needs. 
• Give any network address and subnet mask dtermine the number of subnets and hosts available.
• Determine the range of ip adresses for each subnet implied by a given network address and subnet mask.

Subnetting

• Subnetting refers to the practice of logically subdividing a network address space by extending the string of 1-bits used in the subnet mask of a network. 
• This extension enables you to create multiple subnets within the original network address space.

Example 

•  When the default subnet mask of 255.255.0.0 is used for hosts within the Class B network of 131.107.0.0, the IP addresses 131.107.1.11 and 131.107.2.11 are found on the same subnet, and these hosts communicate with each other by means of a broadcast. However, when the subnet mask is extended to 255.255.255.0, the addresses 131.107.1.11 and 131.107.2.11 are found on different subnets. 
• To communicate with each other, hosts with addresses 131.107.1.11/24 and 131.107.2.11/24 send IP packets to the default gateway, which is then responsible for routing the datagram toward the destination subnet.

Advantages of Subnetting

• Subnetting is often used to accommodate a divided physical topology or to restrict broadcast traffic on a network. 
•  Subnetting includes improved security (by restricting unauthorized traffic behind routers) 
• Subnetting provides simplified administration (by delegating control of subnets to other departments or administrators).

subnetting -   division

                        security

                        administration

Determining Host Capacity for Networks

• For any specific network address, you can determine the quantity of host addresses available within that network by raising 2 to the power of the number of bits in the host ID, and then subtracting 2. 
• For instance, the network address 192.168.0.0/24 reserves 8 bits for the host ID. Therefore, you can determine the number of hosts by calculating 2⁸ – 2, which equals 254. 
• Using Calculator, you can easily compute the value of 2 to any power with the x^y button. This function appears only in Scientific view.

Excluding All-0s and All-1s Host IDs

• combinations of all 0s and all 1s (0 and 7 in the preceding list) cannot be assigned to hosts because these addresses are reserved for other purposes. Specifically, the all-0s host ID is invalid because it is used to specify a network without specifying a host. 
• The all-1s host ID cannot be assigned to a particular host because it is used by IP to broadcast a message to every host on a network. Because these values cannot be assigned to hosts, you must account for these nonusable host IDs by subtracting 2 from 2^x when determining the host capacity of your network.

 

• To determine the number of subnets available within an address space, simply calculate the value of 2^y, where y equals the number of bits in the subnet ID. 
• For example, when the network address space 172.16.0.0/16 is subnetted to /24, 8 bits are reserved for the subnet ID. Therefore, the number of available subnets is 28, or 256. 
• You do not usually need to subtract 2 from this total because most modern routers (including the Routing And Remote Access service in Microsoft Windows NT Server, Microsoft Windows 2000 Server, and Windows Server 2003) can accept a subnet ID made up of all 1s or all 0s.