Topics. Subnetting. The Basics of Subnetting Subnet Mask Computing subnets and hosts Subnet Routing Creating a Subnet Example of Subnetting 1/37

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1 1/37 Subnetting Surasak Sanguanpong Last updated: 27 June 2002 Topics 2/37 The Basics of Subnetting Subnet Mask Computing subnets and hosts Subnet Routing Creating a Subnet Example of Subnetting

2 Addressing without Subnets 3/ A class B Flat Network, more than hosts How to manage? Performance? Addressing with Subnets 4/ A class B subdivided network, smaller groups with routers

3 Subnetwork benefits 5/37 Overall traffic is reduced, performance may improve Smaller networks are easier to manage and troubleshoot Increase the network manager's control over the address space Subnetwork Subdivide on IP network number is an important initial task of network managers Subnet Address Before Subnetting Network ID Host ID 6/37 After Subnetting Network ID Subnet ID Host ID A subnet address is created by borrowing bit from the Host ID and designated it as a Subnet ID field

4 How to assign subnet 7/37 Each class can have different size of subnet field Define physical subnetwork Define individual hosts Network Subnet Host choose appropriate size Class A : 2 to 22 bits Class B : 2 to 14 bits Class C : 2 to 6 bits Subnet Example 8/37 Class B address such as might use its third byte to identify subnet Subnet Network Address Address Range # #2 # #

5 Subnet mask 9/37 subnet mask is a 32 bit number, use to identify a subnet Example : A class B network with 24 bits mask 1 Set the bit covering the network and subnet ID to 1 Network ID Subnet ID Host ID subnet mask= zero bit are used to mask out the host number resulting the network address Masking 10/ & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & A bitwise-and between IP address and subnet mask yields a network address. Note that zeros bit are used to mask out the host number resulting the network address

6 Subnet mask in Prefix format 11/37 The number of routing bits (network and subnet bits) in each subnet mask can also be indicated by the "/n " format / /18 /24 / = /24 Subnet routing 12/37 Traffic is routed to a host by looking bit-wise and results if dest_ip_addr & subnet_mask = = my_ip_addr & subnet_mask send pkt on local network %dest ip addr is on the same subnet else send pkt to router %dest ip addr is on diff subnet

7 Routing 13/37 Hosts and routers perform logical AND to send packets To / / / / has a packet for and determine that it is on other subnetwork The packet is sent to the router The router performs a subnet masking and sends the packet to the destination network Subnet interpretation 14/37 IP Address subnet mask Interpretation host 15.2 on subnet host 3 on subnet host 4th on subnet host 71 on subnet host 2nd on subnet

8 Default Subnet mask 15/37 A default subnet mask : a subnet mask with no subnetting Class A Class B Class C Range of bit 16/37 A default subnet mask : a subnet mask with no subnetting 172 IP 16 Default subnet New subnet Define a subnet mask by extending the network portion to the right, 8 bits in this example

9 Computing subnet mask 17/37 Decimal equivalents of bit patterns Binary mask Octet value Compute Net and host 18/37 How many subnet and host are there with / Network ID Subnet ID Host ID 8 bit subnet ID = 2 8 =256 => 254 subnets 8 bit host ID = 2 8 =256 => 254 hosts per subnet

10 Network and Host relationship 19/37 Sample class C Number of subnet bits 2 Number of subnets created 2 Number of hosts per subnet 62 Total number of hosts 124 Percents used 49% % % % % Subnetting Special Addresses 20/37 Reserved addresses that are not allowed to be assigned to any node NetID any HostID All 0s Example: /24 Subnetwork any All 1s Purpose Subnetwork Address Subnet-directed Broadcast Example: /24 Directed broadcast of the subnetwork

11 Subnet Net Block Diagram No subnetting 2 bits 21/37 Block diagram subnetting class C 3 bits 4 bits Network Address Broadcast Address Contiguous and Noncontiguous mask 22/37 Contiguous subnet mask no intermedite 0 gaps in the subnet mask Noncontiguous subnet mask intermedite 0 gaps in the subnet mask Noncontiguous leads to complex subnetting and routing It is strongly recommend to use contiguous subnet mask

12 Subnet Class A Example 23/37 subnet mask Interpretation network with hosts (default subnet) subnets each with hosts subnets each with hosts subnets each with hosts subnets each with 254 hosts Example : Class A Subnet Address Table IP Address : /16 24/37 SubnetID all 0s #1 #2 #254 Network Address Broadcast Address SubnetID all 1s

13 Class A Subnet with router 25/37 # # to to / subnets each with hosts # to # to Subnet Class B Example 26/37 subnet mask Interpretation network with hosts (default subnet) subnets each with hosts subnets each with 1022 hosts subnets each with 254 hosts subnets each with 2 hosts

14 Example : Class B Subnet Address Table 27/37 IP Address : /24 SubnetID all 0s #1 #2 #254 Network Address Broadcast Address SubnetID all 1s Class B Subnet with router 28/37 # to to / subnets each with hosts #2 # to # to

15 Subnet Class C Example 29/37 subnet mask Interpretation network with 254 hosts (default subnet) subnets each with 62 host subnets each with 30 hosts subnets each with 14 hosts subnets each with 2 hosts Example : Class C Subnet Address Table 30/37 IP Address : /27 SubnetID all 0s #1 #2 #6 Network Address Broadcast Address SubnetID all 1s

16 Class C Subnet with router 31/37 # to to /27 6 subnets each with 30 hosts #2 # to # to Subnet Exercise (1) 32/37 Given IP address , find out the following to yield not more than 256 hosts per subnet net mask=?? start net id =?? end net id=?? #of subnet =??

17 Subnet Exercise (2) 33/37 Given IP address , find out the following to yield not more than 32 hosts per subnet net mask=?? start net id =?? end net id=?? #of subnet =?? Type of Subnetting 34/37 Static Subnetting all subnets in the subnetted network use the same subnet mask pros: simply to implement, easy to maintain cons: wasted address space (consider a network of 4 hosts with wastes 250 IP) Variable Lengh Subnetting the subnets may use different subnet masks pros: utilize address spaces cons: required well-management

18 Problem of Static subnetting /27 used 20 hosts, waste 10 hosts 35/ /27 used 20 hosts, waste 10 hosts /27 used 25 hosts, waste 5 hosts /27 used 25 hosts, waste 5 hosts Inefficient allocation of the address space /27 used 10 hosts, waste 20 hosts /27 used 10 hosts, waste 20 hosts Variable-Length Subnetting /27 used 20 hosts, waste 10 hosts /27 used 20 hosts, waste 10 hosts /27 used 25 hosts, waste 5 hosts /27 used 25 hosts, waste 5 hosts /28 used 10 hosts, waste 4 hosts 36/37 General Idea of VLSM A small subnet with only a few hosts needs a subnet mask that accommodate only few hosts A subnet with many hosts need a subnet mask to accommodate the large number of hosts /28 unused subnet Available 14 hosts /28 used 10 hosts, waste 4 hosts /28 unused subnet Available 14 hosts

19 VLSM - An Example three different VLSM of CPC RDI / point-to-point link CPE