Chapter 6 Subnet Mask

Class C network(285)

1. The class C network shown in Figure 6-15 is partitioned into four subnets. 2. It takes 2 bits to provide four possible subnets; therefore, 2 bits are borrowed from the host bits. 3. This means the process of creating the 4 subnets reduces the number of bits available for host IP addresses.

calculating the number of subnets (next step)

1. The next step is to determine the subnet mask required for creating the four subnets. 2. Recall that creating the four subnets required borrowing 2 host bits. 3. The two most significant bit (MSB) positions, borrowed from the host and network portion of the IP address, must be included in the subnet mask selection.

Subnets(fig 6-14)(285)

1. are created by borrowing bits from the host portion of the IP address. 2. The network portion of the IP address and the new subnet bits are used to define the new sub net. 3. Routers use this information to properly forward data packets to the proper sub net.

Place value (288)

1.The subnet mask also identifies the subnet bits. 2. The two MSBs were borrowed from the host bits; therefore, the last octet of the subnet mask will be where the 1 indicates this place is used for the subnet mask and the x means that the place value is left for the host address. Summing the two bit position values that have a 1 yields 128 + 64 = 192. The 192 is placed in the last octet of the subnet mask. The complete sub net mask is 255.255.255.192.

calculating the number of subnets

The equations for calculating the number of subnets created and the number of hosts/subnet are provided in Equations 6-1 and 6-2. # of subnets created 2x # of hosts/subnet [Equation 6-1] [Equation 6-2] where x = # of bits borrowed from the host bits y = # host bits for the class of network (A = 24, B = 16, C = 8) Breaking down the 192.168.12.0 network into four subnets requires borrowing two host bits. Therefore, x = 2, and because this is a class C network, y = 8. x = 2 (the number of bits used from the host) y = 8 (number of bits for a class C network) Applying these values to equation 6-2 yields The number of sub nets created = 2x = 2(2)= 4 The number of hosts/subnet = 2(Y-x) = 2(8-2)= 64

CIDR-Subnet Mask Conversion

A network address and the subnet mask of 192.168.12.0 255.255.252.0 can be written in CIDR notation as 192.168.12.0122. Table 6-16 provides the CIDR for the most common sub net masks

Prefix Length notation

Another shorthand technique for writing the sub net mask except class boundaries are not being crossed

selecting a subnet mask(pg 290)

Another way to look at selecting a subnet mask is by specifying how many usable hosts are available to be assigned in a subnet. For example, assume that 62 usable host addresses are to be available in a sub net. Assume this is for a class C network. Using Equation 6-2,

Breakdown of Ip Address(286)

Assume that the network has an IP address of 192.168.12.0. The 2 bits are borrowed from the host portion of the IP address to create the 4 subnets. The class C network has 24 network bits and 8 host bits. Then 2wo bits are borrowed from the host address to create the 4 subnets. The network plus subnet portion of the IP address is now 24 + 2, or 26 bits in length, and the host portion is now 6 bits. The breakdown of the 32-bit IP address is shown in Figure 6-16.

Example 6-9(pg 291)

Determine the subnet mask required for the router-to-router link shown in Figure 6- 18 if only two host addresses are required for this link.

Binary and Decimal Equivalents for the subnets Broadcast address

Each subnet will also have its own broadcast address. The broadcast address for the subnet is used to broadcast packets to all hosts in the subnet. (Note: All host bits are set to 1 for a broadcast.) Table 6-13 shows the binary and decimal equivalents for the subnet's broadcast address

Example 6-10

Explore what happens if the boundary in IP addresses for Class C subnets is crossed. For this example, the subnets have IP addresses of

CIDR example

For example, the Class C size subnet mask 255.255.255.0 is listed in CIDR notation as 124. This indicates the 24 bits are set to a 1. A Class B size subnet is written as /16, and a Class A subnet is written as /8. CIDR can also be used to represent subnets that identify only part of the octet bits in an IP address. For example, a subnet mask of 255.255.192.0 is written in CIDR as /18. The /18 comes from the 18 bits that are set to a 1 as shown:

Example 6-8 demonstrate sub net mask selection in a network

Given a network address of 10.0.0.0, divide the network into 8 subnets. Specify the subnet mask, the network and broadcast addresses, and the number of usable hosts/subnet. Solution: Creating 8 subnets requires borrowing 3 host bits; therefore, x = 3. This is a class A network, so y = 24

Network and Broadcast Addresses for the Four Subnets of the 192.168.12.0 Network

Given this information, the network and broadcast address can be defined for the four subnets of the 192.168.12.0 network. Table 6-14 provides these addresses for the four subnets.

supernetting

Supernetting allows multiple networks to be specified by one subnet mask. In other words, the class boundary could be overcome. Supernetting required a simpler way to indicate the sub net mask

MSB positions(3) pg 290

The 224 in Example 6-8 comes from setting the subnet mask to select the three MSB positions in the host portion of the address, as shown in Figure 6-17.

Supernets

The grouping of two or more class networks together; also called CIDR blocks. 1. The group of four IP addresses from 192.168.76.0 to 192.168.79.0 with a CIDR ofl22 is a supernet. 2. The supernet uses a CIDR subnet mask (/22) that is shorter than the number of network bits for Class C network (/24). 3. Another example of a supernet is 172.16.0.0/12. 172.16.0.0 is a Class B address, and the CIDR subnet mask (/12) is less than the 16 bits for the network portion of a Class B address.

CIDR BLock

The grouping of two or more class networks together; also called supernetting CIDR blocks are used to break down the class barriers in IP addressing. For example, two Class C networks (192.168.78.0124 and 192.168.79.0124) can be grouped together as one big sub net. These two networks can be grouped together by modifying the 124 CIDR number to 123. This means that one bit has been borrowed from the network address bits to combine the two networks into one supernet.

CIDR

The technique developed is called classless interdomain routing (CIDR). CIDR (pronounced "cider") notation specifies the number of bits set to a 1 that make up the subnet mask.

Binary and Decimal Equivalents for the Subnet's Network Address

The two subnet bits create four subnets, and each subnet has its own network address. The network addresses are used to route data packets to the correct subnet. Table 6-12 shows the four subnet addresses listed in both binary and decimal format. (Note: The six host bits are all set at 0 in a subnet's network address.)

Subnet Mask

identifies which bits in the IP address are to be used to represent the network/subnet portion of an IP address.

Subnetting

is a technique used to break down (or partition) networks into subnets. The subnets are created through the use of sub net masks.

classful

means that the IP addresses and subnets are within the same network. The problem with classful addressing is that there is a lot of unused IP address space. For example, a class A IP network has more than 16 million possible host addresses. A Class B network has more than 65,000 host addresses, but the fact is that only a limited number of Class A and B address space has been allocated for Internet use.