1/37 Subnetting Surasak Sanguanpong nguan@ku.ac.th http://www.cpe.ku.ac.th/~nguan Last updated: 27 June 2002 Applied Network Research Group Department of Computer Engineering, Kasetsart University 2/37 Topics � The Basics of Subnetting � Subnet Mask � Computing subnets and hosts � Subnet Routing � Creating a Subnet � Example of Subnetting Applied Network Research Group Department of Computer Engineering, Kasetsart University
3/37 Addressing without Subnets 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? Applied Network Research Group Department of Computer Engineering, Kasetsart University 4/37 Addressing with Subnets 172.16.1.2 172.16.1.3 172.16.2.2 172.16.2.3 172.16.1.0 172.16.2.0 172.16.4.2 172.16.4.3 172.16.3.2 172.16.3.3 172.16.3.0 172.16.4.0 A class B “subdivided network”, smaller groups � with routers Applied Network Research Group Department of Computer Engineering, Kasetsart University
5/37 Subnetwork benefits Smaller networks are easier to manage Increase the network Overall traffic is and troubleshoot manager's control over reduced, performance the address space may improve Subnetwork Subdivide on IP network number is an important initial task of network managers Applied Network Research Group Department of Computer Engineering, Kasetsart University 6/37 Subnet Address Before Subnetting Network ID Host ID 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University
7/37 How to assign subnet Each class can have different size of subnet field � Define physical subnetwork Define individual hosts Network Subnet Host choose appropriate size Class A : Class C : 2 to 22 bits Class B : 2 to 6 bits 2 to 14 bits Applied Network Research Group Department of Computer Engineering, Kasetsart University 8/37 Subnet Example � 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. 0 172.16.1.1-172.16.1.254 #2 172. 16. 2. 0 172.16.2.1-172.16.2.254 #3 172. 16. 3. 0 172.16.3.1-172.16.3.254 #254 172. 16. 254. 0 172.16.254.1-172.16.254.254 Applied Network Research Group Department of Computer Engineering, Kasetsart University
9/37 Subnet mask subnet mask is a 32 bit number, use to identify a subnet � Example : A class B network with 24 bits mask Network ID Subnet ID Host ID 1 1111 1111 1111 1111 1111 1111 0000 0000 Set the bit covering the network and subnet ID to 1 255. 255. 255. 0. subnet mask= 255.255.255.0 2 zero bit are used to mask out the host number resulting the network address Applied Network Research Group Department of Computer Engineering, Kasetsart University 10/37 Masking & 255.255.255.0 172.16.4.2 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University
11/37 Subnet mask in Prefix format 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 /8 1111 1111 0000 0000 0000 0000 0000 0000 255.255.192.0 /18 1111 1111 1111 1111 1100 0000 0000 0000 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 /24 255.255.255.240 1111 1111 1111 1111 1111 1111 1111 0000 /28 172.16.0.0 255.255.255.0 = 172.16.0.0/24 Applied Network Research Group Department of Computer Engineering, Kasetsart University 12/37 Subnet routing 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University
13/37 Routing 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 1 172.16.1.0/24 172.16.2.0/24 2 172.16.4.2 172.16.4.3 172.16.3.2 172.16.3.3 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University 14/37 Subnet interpretation 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University
15/37 Default Subnet mask A default subnet mask : a subnet mask with no � subnetting Class A 255.0.0.0 1111 1111 0000 0000 0000 0000 0000 0000 Class B 255.255.0.0 1111 1111 1111 1111 0000 0000 0000 0000 Class C 255.255.255.0 1111 1111 1111 1111 1111 1111 0000 0000 Applied Network Research Group Department of Computer Engineering, Kasetsart University 16/37 Range of bit A default subnet mask : a subnet mask with no � subnetting IP 172 16 0 0 Default subnet 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University
17/37 Computing subnet mask Decimal equivalents of bit patterns Binary mask Octet value 128 64 32 16 8 4 2 1 1 0 0 0 0 0 0 0 128 1 1 0 0 0 0 0 0 192 1 1 1 0 0 0 0 0 224 1 1 1 1 0 0 0 0 240 1 1 1 1 1 0 0 0 248 1 1 1 1 1 1 0 0 252 1 1 1 1 1 1 1 0 254 1 1 1 1 1 1 1 1 255 Applied Network Research Group Department of Computer Engineering, Kasetsart University 18/37 Compute Net and host How many subnet and host are there with � 172.16.0.0/24 255. 255. 255. 0. 1111 1111 1111 1111 1111 1111 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 Applied Network Research Group Department of Computer Engineering, Kasetsart University
19/37 Network and Host relationship � Sample class C Number of Number of Number of Total Percents subnet subnets hosts number of used bits created per subnet hosts 2 2 62 124 49% 3 6 30 180 71% 4 14 14 196 77% 5 30 6 180 71% 6 62 2 124 49% Applied Network Research Group Department of Computer Engineering, Kasetsart University 20/37 Subnetting Special Addresses Reserved addresses that are not allowed to be assigned to any node NetID HostID Purpose any All 0s Subnetwork Address Example: 172.16.2.0/24 Subnetwork 172.16.2.0 any All 1s Subnet-directed Broadcast Example: 172.16.2.255/24 Directed broadcast of the subnetwork 172.16.2.0 Applied Network Research Group Department of Computer Engineering, Kasetsart University
21/37 Subnet Net Block Diagram No subnetting 2 bits � Block diagram subnetting class C 3 bits 4 bits Network Address Broadcast Address Applied Network Research Group Department of Computer Engineering, Kasetsart University 22/37 Contiguous and Noncontiguous mask no intermedite 0 gaps in the subnet mask Contiguous subnet mask 1111 1111 1111 1111 1111 1111 0000 0000 intermedite 0 gaps in the subnet mask Noncontiguous subnet mask 1111 1111 1111 1111 0001 1111 0000 0000 Noncontiguous leads to complex subnetting and routing It is strongly recommend to use contiguous subnet mask Applied Network Research Group Department of Computer Engineering, Kasetsart University
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