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# Subnetting

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### Subnetting

1. 1. SubnetsRouting within an Organization
2. 2. 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
3. 3. Subnets Subnet 1 Subnet 2 Subnet 3 Subnet 4
4. 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
5. 5. 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
6. 6. 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
7. 7. 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
8. 8. Routing with Subnets 17710 = ER2101100012 Subnet 1137.207.177.81 00 (137.207.0.0) 01 ER1 IR Subnet 2 10 Subnet 3 11 ER3 Subnet 4
9. 9. Routing with Subnets 17710 = ER2 12810 =101100012 Subnet 1 100000002 00 137.207.177.81 (137.207.128.0) 01 ER1 IR Subnet 2 10 Subnet 3 11 ER3 Subnet 4
10. 10. Routing with Subnets ER2 Subnet 1 00 01ER1 IR Subnet 2 10 Subnet 3 137.207.177.81 (137.207.177.81) 11 ER3 Subnet 4
11. 11. 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
12. 12. Multi-Level Subnets Layer 1 Layer 2 Subnet 1 Subnet 2aNetwork 1 Subnet 2 Subnet 2b Subnet 3
13. 13. 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)
14. 14. Common Class B Subnet MasksSubnet Mask # of Subnets255.255.192.0 4 (22)(19210 = 110000002)255.255.240.0 16 (24)(24010 = 111100002)255.255.252.0 64 (26)(25210 = 111111002)• Class A and C subnet masks would be similar
15. 15. 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)