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Subnets

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.

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Subnets

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  1. Subnets Routing within an Organization

  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. Subnets Subnet 1 Subnet 2 Subnet 3 Subnet 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. 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. 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. 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. Routing with Subnets ER2 Subnet 1 17710 = 101100012 00 137.207.177.81 (137.207.0.0) Subnet 2 ER1 01 IR 10 Subnet 3 11 ER3 Subnet 4

  9. Routing with Subnets ER2 Subnet 1 17710 = 101100012 12810 = 100000002 00 137.207.177.81 (137.207.128.0) Subnet 2 ER1 01 IR 10 Subnet 3 11 ER3 Subnet 4

  10. Routing with Subnets ER2 Subnet 1 00 Subnet 2 ER1 01 IR 10 Subnet 3 137.207.177.81 (137.207.177.81) 11 ER3 Subnet 4

  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. Multi-Level Subnets Layer 2 Layer 1 Subnet 1 Subnet 2a Network 1 Subnet 2 Subnet 2b Subnet 3

  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 (19210 = 110000002)

  14. Common Class B Subnet Masks • Class A and C subnet masks would be similar

  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)

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