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)
- Organization has Class B address (16 host ID bits) with network ID: 150.100.0.0
- 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)
- 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
- IP address = 10010110 01100100 00001100 10000001
Subnet Example

The subnetting can hide the configuration details from outside
Routing with Subnetworks
- IP layer in hosts and routers maintain a routing table
- 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
- 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
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