4. Subnetting #

🔒 Recording

IP Address Problem #

• Class A and B networks are too big
  • Very few LANs have close to 64,000 hosts
  • For electrical/LAN limitations, performance, or administrative reasons
• Address space depletion
  • Running out of Class A and B addresses, since Class C is too small for most domains
• Clsas B sparsely populated, but people refuse to give it back
• Large forwarding tables
  • 2 million possible Class C groups

Solutions #

• Short-term solutions:
  • Subnetting
  • Classless Inter-Domain Routing (CIDR), RFC 1518
  • New allocation policy (RFC 2050)
  • Private IP addresses set aside for intranets
• Long-term solution:
  • IPv6, with much bigger address space (128-bit)

Subnet and Classless Addressing #

• Not part of the original scheme
• Invented to prevent address exhaustion
• Allows the boundary between prefix and suffix to occur on an arbitrary bit boundary
• Requires auxiliary information to indentify the boundary

Subnetting #

• Goal: to extend address space
• Subnet addressing introduces another hierarchical level
• Transparent to remote networks
• Simplifies management of multiplicity of LANs
• Technique:
  • Assign single network prefix to site
  • Divide suffix into two parts: network at site and host
• Typical use: divide Class B addresses

Address Mask #

• Accompanies the IP address
• 32-bit binary value
• Specifies prefix/suffix boundary
  • 1s cover prefix (network)
  • 0s cover suffix (host)
• Masking used to find subnet number
• Performing a bitwise logical AND operation between the IP address and the subnet mask results in the Network Address or Number
  • For example: the Class B mask is 255.255.0.0

Subnetting (Continued) #

• Take a network address and break it up into subnets that can be assigned to individual physical networks
• Define a subnet mask to help create a new level of hierarchy in the addressing scheme
• The bitwise AND of the subnet mask with the full address gives the subnet number
• Group physically close networks to share a single Network ID
• Subnetting is not visible to the outside world. An organisation decides internally how to implement subnetting

Subnetting Example #

• Assume an organisation was assigned address 150.100
• Assume < 100 hosts per subnet
• So we need 7 host bits
• Therefore network mask is 11111111 11111111 11111111 10000000 or 255.255.255.128
• Apply the subnet mask to IP addresses to find corresponding subnet

Example: find subnet for 150.100.12.176

IP Address : 10010110 01100100 00001100 10110000
Mask       : 11111111 11111111 11111111 10000000
AND        : 10010110 01100100 00001100 10000000
Subnet     : 150.100.12.128

Calculating Available Subnets/Hosts #

• To calculate the number of subnets (avoid the zero and broadcast subnet) or nodes, use the formula: 2ⁿ - 2, where n is the number of bits in either field
• To calculate the number of subnets when using Classless routing protocol, RIP V2, EIGRP, or OSPF, use: 2ⁿ
• Number of subnets * number of nodes per subnet = total number of nodes in class

Example 1 #

IP Address : 10001100.10110011.11011100.11001000    140.179.220.200
Mask       : 11111111.11111111.11100000.00000000    255.255.224.000
AND        : 10001100.10110011.11000000.00000000    140.179.192.000

• 3-bit subnet mask is used
• Subnet address is calculated to be 140.179.192.000, as shown above
• Number of subnets = 2³ - 2 = 6

Example 2 #

• You are assigned a Class C network number: 200.133.175.0
• You want to utilise this network across multiple small groups within your organisation
• Number of nodes you’ll get without subnetting: 2⁸ - 2 = 254
• You decide to break the network into 14 smaller subnets
  • To get 14 subnets, you need a subnet mask of 4 bits
  • Hosts per subnet: 2⁴ - 2 = 14

Routing with Subnets #

• IP layer in hosts and routers maintain a routing table
• Originating host: to send an IP packet, consult routing table:
  • If destination host is in the same network, send packet directly uing 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, then forwards packet

Routing Table Example #

Additional Points #

• The subnet mask does not need to align on byte boundaries
• It is possible to put multiple subnets on the same physical network, but hosts may then have to go through a router to talk to each other
• From outside the subnetted domain, the whole thing is viewed as a single network. For this reason, subnets should be kept geographically close