1/37 Subnetting Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan 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
Addressing without Subnets 3/37 172.16.1.2 172.16.1.3 172.16.2.1 172.16.254.254 172.16.0.0 A class B Flat Network, more than 65000 hosts How to manage? Performance? Addressing with Subnets 4/37 172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3 172.16.1.0 172.16.2.0 172.16.3.2 172.16.3.3 172.16.4.2 172.16.4.3 172.16.3.0 172.16.4.0 A class B subdivided network, smaller groups with routers
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
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 172.16.0.0 might use its third byte to identify subnet Subnet Network Address Address Range #1 172.16.1.1-172.16.1.254 172. 16. 1. 0 #2 #3 172. 16. 2. 0 172. 16. 3. 0 172.16.2.1-172.16.2.254 172.16.3.1-172.16.3.254 #254 172. 16. 254. 0 172.16.254.1-172.16.254.254
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 0000 0000 255. 255. 255. 0. 2 subnet mask= 255.255.255.0 zero bit are used to mask out the host number resulting the network address Masking 10/37 172.16.4.2 & 255.255.255.0 1 0 1 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 1 0 1 0 1 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 172.16.4.0 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
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. 255.0.0.0 0000 0000 0000 0000 0000 0000 /8 255.255.192.0 255.255.255.0 255.255.255.240 1100 0000 0000 0000 0000 0000 1111 0000 /18 /24 /28 172.16.0.0 255.255.255.0 = 172.16.0.0/24 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
Routing 13/37 Hosts and routers perform logical AND to send packets 172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3 To 172.16.4.2 1 172.16.1.0/24 172.16.3.2 172.16.3.3 2 172.16.2.0/24 172.16.4.2 172.16.4.3 3 172.16.3.0/24 172.16.4.0/24 172.16.1.3 has a packet for 172.16.4.2 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 15.20.15.2 255.255.0.0 host 15.2 on subnet 15.20.0.0 130.122.34.3 255.255.255.192 host 3 on subnet 130.122.34.0 130.122.34.132 255.255.255.192 host 4th on subnet 130.122.34.128 158.108.2.71 255.255.255.0 host 71 on subnet 158.108.2.0 200.190.155.66 255.255.255.192 host 2nd on subnet 200.190.155.64
Default Subnet mask 15/37 A default subnet mask : a subnet mask with no subnetting Class A 255.0.0.0 0000 0000 0000 0000 0000 0000 Class B 255.255.0.0 0000 0000 0000 0000 Class C 255.255.255.0 0000 0000 Range of bit 16/37 A default subnet mask : a subnet mask with no subnetting 172 IP 16 Default subnet 0 0 255 255 0 0 New subnet 255 255 255 0 Define a subnet mask by extending the network portion to the right, 8 bits in this example
Computing subnet mask 17/37 Decimal equivalents of bit patterns Binary mask Octet value 128 64 32 16 8 4 2 1 1 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 1 0 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 1 0 0 0 1 1 1 1 1 1 0 0 1 1 1 1 1 1 1 0 1 1 1 1 1 1 1 1 128 192 224 240 248 252 254 255 Compute Net and host 18/37 How many subnet and host are there with 172.16.0.0/24 255. 255. 255. 0. 0000 0000 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
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% 3 6 30 180 71% 4 14 14 196 77% 5 30 6 180 71% 6 62 2 124 49% Subnetting Special Addresses 20/37 Reserved addresses that are not allowed to be assigned to any node NetID any HostID All 0s Example: 172.16.2.0/24 Subnetwork 172.16.2.0 any All 1s Purpose Subnetwork Address Subnet-directed Broadcast Example: 172.16.2.255/24 Directed broadcast of the subnetwork 172.16.2.0
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 0000 0000 Noncontiguous subnet mask intermedite 0 gaps in the subnet mask 0001 1111 0000 0000 Noncontiguous leads to complex subnetting and routing It is strongly recommend to use contiguous subnet mask
Subnet Class A Example 23/37 subnet mask Interpretation 255.0.0.0 1 network with 1677214 hosts (default subnet) 255.255.0.0 254 subnets each with 65534 hosts 255.255.128.0 510 subnets each with 32768 hosts 255.255.192.0 1022 subnets each with 16382 hosts 255.255.255.0 65534 subnets each with 254 hosts Example : Class A Subnet Address Table IP Address : 10.0.0.0/16 24/37 SubnetID all 0s 10.0.0.0 10.0.0.1 10.0.255.254 10.0.255.255 #1 #2 #254 Network Address 10.1.0.0 10.1.0.1 10.1.255.254 10.1.255.255 10.2.0.0 10.2.0.1 10.2.255.254 10.2.255.255 10.254.0.0 10.254.0.1 10.254.255.254 10.254.255.255 Broadcast Address 10.255.0.0 10.255.0.1 10.255.255.254 10.255.255.255 SubnetID all 1s
Class A Subnet with router 25/37 #1 10.1.0.0 #2 10.1.0.1 to 10.1.255.254 10.2.0.0 10.2.0.1 to 10.2.255.254 10.0.0.0/16 254 subnets each with 65534 hosts #3 10.3.0.0 10.3.0.1 to 10.3.255.254 #254 10.255.0.0 10.255.0.1 to 10.255.255.254 Subnet Class B Example 26/37 subnet mask Interpretation 255.255.0.0 1 network with 65534 hosts (default subnet) 255.255.192.0 2 subnets each with 16382 hosts 255.255.252.0 62 subnets each with 1022 hosts 255.255.255.0 254 subnets each with 254 hosts 255.255.255.252 16382 subnets each with 2 hosts
Example : Class B Subnet Address Table 27/37 IP Address : 176.16.0.0 /24 SubnetID all 0s 172.16.0.0 172.16.0.1 172.16.0.254 172.16.0.255 #1 #2 #254 Network Address 172.16.1.0 172.16.1.1 172.16.1.254 172.16.1.255 172.16.2.0 172.16.2.1 172.16.2.254 172.16.2.255 172.16.254.0 172.16.254.1 176.16.254.254 176.16.254.255 Broadcast Address 176.16.255.0 176.16.255.1 176.16.255.254 176.16.255.255 SubnetID all 1s Class B Subnet with router 28/37 #1 172.16.1.0 172.16.1.1 to 172.16.1.254 172.16.2.0 172.16.2.1 to 172.16.2.254 172.16.1.0/24 254 subnets each with 65534 hosts #2 #3 172.16.3.0 172.16.3.1 to 172.16.3.254 #254 172.16.254.0 172.16.254.1 to 172.16.254.254
Subnet Class C Example 29/37 subnet mask Interpretation 255.255.255.0 1 network with 254 hosts (default subnet) 255.255.255.192 2 subnets each with 62 host 255.255.255.224 6 subnets each with 30 hosts 255.255.255.240 14 subnets each with 14 hosts 255.255.255.252 62 subnets each with 2 hosts Example : Class C Subnet Address Table 30/37 IP Address : 192.68.0.0 /27 SubnetID all 0s 192.68.0.0 192.68.0.1 192.68.0.30 192.68.0.31 #1 #2 #6 Network Address 192.68.0.32 192.68.0.33 192.68.0.62 192.68.0.63 192.68.0.64 192.68.0.65 192.68.0.94 192.68.0.95 192.68.0.192 192.68.0.193 192.68.0.222 192.68.0.223 Broadcast Address 192.68.0.224 192.68.0.225 192.68.0.254 192.68.0.255 SubnetID all 1s
Class C Subnet with router 31/37 #1 192.68.0.32 192.68.0.33 to 192.68.0.62 192.68.0.64 192.68.0.65 to 192.68.0.94 192.68.0.0/27 6 subnets each with 30 hosts #2 #3 192.68.0.96 192.68.0.97 to 192.68.0.126 #6 192.68.0.192 192.68.0.193 to 192.68.0.222 Subnet Exercise (1) 32/37 Given IP address 161.200, find out the following to yield not more than 256 hosts per subnet net mask=?? start net id =?? end net id=?? #of subnet =??
Subnet Exercise (2) 33/37 Given IP address 192.150.251, 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 255.255.255.0 wastes 250 IP) Variable Lengh Subnetting the subnets may use different subnet masks pros: utilize address spaces cons: required well-management
Problem of Static subnetting 192.68.0.32/27 used 20 hosts, waste 10 hosts 35/37 192.68.0.64/27 used 20 hosts, waste 10 hosts 192.68.0.96/27 used 25 hosts, waste 5 hosts 192.68.0.128/27 used 25 hosts, waste 5 hosts Inefficient allocation of the address space 192.68.0.192/27 used 10 hosts, waste 20 hosts 192.68.0.224/27 used 10 hosts, waste 20 hosts Variable-Length Subnetting 192.68.0.32/27 used 20 hosts, waste 10 hosts 192.68.0.64/27 used 20 hosts, waste 10 hosts 192.68.0.96/27 used 25 hosts, waste 5 hosts 192.68.0.128/27 used 25 hosts, waste 5 hosts 192.68.0.192/28 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 192.68.0.208/28 unused subnet Available 14 hosts 192.68.0.224/28 used 10 hosts, waste 4 hosts 192.68.0.240/28 unused subnet Available 14 hosts
VLSM - An Example three different VLSM of 172.16.0.0 CPC 255.255.255.0 RDI 255.255.255.0 37/37 255.255.255.252 255.255.255.252 255.255.255.252 point-to-point link 255.255.255.252 CPE 255.255.255.192