Subnets Routing within an Organization Subnet Subnets are

Subnets Routing within an Organization Subnet Subnets are a subset of the entire network Networks can be divided into subnets Subnets can be divided into subnets Each subnet is treated as a separate network A subnet can be a WAN or LAN

Subnets Subnet 1 Subnet 2 Subnet 3 Subnet 4 Subnet Addresses Typical (classed) IP addresses (e.g. 137.207.32.2) have two parts:

A network ID (e.g. 137.207.0.0) A host ID (e.g. 0.0.32.2) A subnetted network will divide the IP address differently Part of the host ID will be used to specify the subnet number The network ID and subnet portion of the host ID can be considered the subnet ID Subnet Addresses

For example, if we want to divide a network into 4 subnets, we can use the following scheme: 4 subnets can be represented with 2 bits (22 = 4) For a class B address, which already uses 16 bits for the network portion, the address would use 18 bits for the subnet portion External Routing with Subnets Subnetting is transparent outside an organization e.g.

If subnetting is used in the University, routers outside the University will not consider subnetting at all Thus, subnetting is only relevant within an organization Internal Routing with Subnets When a packet enters a network with subnetting, the routers will behave differently A route may use the subnet ID (network portion + subnet portion, followed by zeroes) to determine which route to take

Routing with Subnets ER2 17710 = 101100012 Subnet 1 00 137.207.177.81 (137.207.0.0) ER1 IR 01

Subnet 2 10 Subnet 3 11 ER3 Subnet 4 Routing with Subnets 17710 = 101100012 ER2 12810 = 100000002 137.207.177.81

(137.207.128.0) ER1 IR Subnet 1 00 01 Subnet 2 10 Subnet 3 11 ER3 Subnet 4

Routing with Subnets ER2 Subnet 1 00 ER1 IR 01 Subnet 2 10 137.207.177.81 (137.207.177.81)

Subnet 3 11 ER3 Subnet 4 Multi-Level Subnets A subnet can be divided into subnets If the first layer of subnetting used 2 bits for the subnet portion, a second layer can be used to subnet within each subnet

Using more bits from the host portion of the address Multi-Level Subnets Layer 1 Layer 2 Subnet 1 Subnet 2a Network 1 Subnet 2 Subnet 2b Subnet 3 Calculating Subnet IDs

Calculating the subnet ID is very much the same process as calculating a network ID (net ID) The difference is that typically net IDs are calculated using one of the three standard subnet masks: 255.0.0.0 (Class A) 255.255.0.0 (Class B) 255.255.255.0 (Class C) A subnet ID is calculated using a non-standard subnet mask e.g. 255.255.192.0 (192 10 = 110000002)

Common Class B Subnet Masks Subnet Mask # of Subnets 255.255.192.0 (19210 = 110000002) 4 (22) 255.255.240.0 (24010 = 111100002) 16 (24) 255.255.252.0 (25210 = 111111002)

64 (26) Class A and C subnet masks would be similar CIDR Notation A convenient way to specify this kind of subnet mask is CIDR addressing e.g. 137.207.32.2/255.255.0.0: 137.207.32.2/16 e.g. 24.1.2.3/255.0.0.0: 24.1.2.3/8

The number after the / is the number of bits that are 1s in the subnet mask 137.207.177.81/255.255.240.0: 137.207.177.81/20 (16 bits for network ID + 4 bits for subnetting)

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