Subnetting

Subnet Addressing

• To better utilize IP address
• Subnet addressing introduces another hierarchical(分层) level
• Transparent to remote networks
• Simplifies management of multiplicity(多样性) of LANs
• Masking used to find subnet number(By devide Host ID into Subnet ID and Host ID)
  

Subnetting Scheme(体系)

1. Organization has Class B address (16 host ID bits) with network ID: 150.100.0.0
2. Create subnets with up to 100 hosts each

• We need one hundred hosts, so we need at least 7 bit(2^7=128>100)for host ID.
• 7 bits sufficient for each subnet (IP mask 7 bits)
• 16-7 = 9 bits for** subnet ID** (2^9 – 2 = 510 subnets)

3. Apply subnet mask to IP addresses to help outside packet find corresponding subnet

• Example: Find subnet for 150.100.12.176
• Theory: We use "and operation" to get the subnet ID, compare every bit
• IP address = 10010110 01100100 00001100 10110000
• Mask = 11111111 11111111 11111111 10000000 (7 0s)
• AND = 10010110 01100100 00001100 10000000
• Subnet = 150.100.12.128
• Subnet address used by routers within organization

Subnet Range

Given the subnet 150.100.12.128

• IP address 150.100.12.128 is used to identify the subnetwork
• IP address 150.100.12.255 is used to broadcast packets in the subnet
• Range of the subnet IP address is between
  • IP address = 10010110 01100100 00001100 10000001
    to
    IP address = 10010110 01100100 00001100 11111110
    That is, 150.100.12.129 to 150.100.12.254

Subnet Example

• The subnetting can hide the configuration details from outside

Routing with Subnetworks

1. IP layer in hosts and routers maintain a routing table
2. Originating host(源主机): To send an IP packet, consult routing table

• If destination host is in same network, send packet directly using appropriate network interface
• Otherwise, send packet indirectly; typically, routing table indicates a default router

3. Router: Examine IP destination address in arriving packet

• If destination IP address not own, router consults routing table to determine next-hop and associated network interface & forwards packet