Introduction to Computer Networking IP Addressing and subnetting M Clements

Last Week . . . . Virtual week – multiple choice assignment Reading of Network Media notes Last time we met in person…. LAN, MAN, WAN, SAN each suit a particular group’s data requirement Many devices exist for networking Hubs, switches, routers, modems, cables etc. All have a maximum bandwidth BW is measured in bps Mbps Gbps etc. Throughput is moderated by real world factors 27/04/2017 ITCN

This Week. . . . IP addressing Reserved addresses Network identity Host identity Subnetworking Subnet masks 27/04/2017 ITCN

Internet Layer - IP Addresses Hierarchical addressing scheme 32 bit address Dotted decimal notation Used for addressing and routing Divided into 2 main sections Network identity Host identity 27/04/2017 ITCN

IP version 4 4 bytes divided into 4 sections Each section can range from 0 to 255 If any portion of the IP address contains a number greater than 255 it is ILEGAL ! Presented with dots in between 4 bytes e.g. 193.60.61.243 11000001.00111100.00111101.11110011 27/04/2017 ITCN

For subnetting use BINARY Always work in Binary All solutions can be discovered in Binary Eventually you will see shortcuts Use Binary first It will take time to understand Practice makes perfect 27/04/2017 ITCN

Address Classes (a history lesson) Classes A, B, C, D, E were created IP numbers that began with : 1 to 126 are class A addresses 128 to 191 are class B addresses 192 to 223 are class C addresses 224 to 239 are class D addresses 240 to 254 are class E addresses. 27/04/2017 ITCN

Exercise 1 – ‘class’ identification To which class do the following IP numbers legally belong ? 193.45.67.123 22.23.24.25 131.74.59.12 210.217.12,98 193.260.50.17 27/04/2017 ITCN

Class C addresses First 3 bytes is network portion Last byte for hosts on network How many hosts can class C have ? A 27/04/2017 ITCN

Class B addresses First 2 bytes is network portion Last 2 bytes for hosts on network How many hosts can class B have ? A 27/04/2017 ITCN

Class A addresses First byte is network portion last 3 bytes for hosts on network How many hosts can class A have ? A 27/04/2017 ITCN

The end of Address class system Classes wasted addresses through inflexibility A better way of dividing network space was sought This was known as Variable Length Subnet Masking or VLSM Now we have to explicitly show the boundary of the host and network portion of an address 27/04/2017 ITCN

Caveat to classless addressing Many people and textbooks still refer to the address class system No networks EVER use the class system in a production environment Be aware of the translations between the classful and classless system Remember ALWAYS to use the classless system despite the terminology of the question 27/04/2017 ITCN

Subnet Mask identifies the boundary Sometimes called the Extended Network Prefix Identifies the two sections of the IP address, network and host fields Used by routers to work out the network a particular host belongs to Written either as dotted decimal or slash notation 27/04/2017 ITCN

Subnet Mask – two notations Dotted decimal is one of the notations 255.255.255.0 In binary this is 11111111.11111111.11111111.00000000 Slash is the second notation Count the number of ‘1’s There are 24 ‘1’s So another way of expressing this subnet mask is /24 27/04/2017 ITCN

Network (or wire) Address This is the identity of the network and is used by routers to deliver packets across networks No host is ever given this address To discover the network address, convert IP address from decimal to binary Replace the host section with 0s Convert back to decimal 27/04/2017 ITCN

Finding the Network Identity 175.13.155.121 /16 10101111.00001101. 10011011. 01111001 What address class was this ? Which is the host field ? Replace with 0s Convert back to dotted decimal A 27/04/2017 ITCN

Broadcast Address RFC 919 Each network needs a broadcast address No host is ever given this address A broadcast can message all hosts in a LAN Useful when a host needs to find information without knowing exactly what other host can supply it e.g. ARP When a host wants to provide information to a large set of hosts in a timely manner 27/04/2017 ITCN

Finding the Broadcast Number Substitute the host section of the binary version of the IP address with 1s Convert back to dotted decimal What is the broadcast address for the IP 175.13.155.121 /16 A 27/04/2017 ITCN

Reserved addressing numbers Host bits set to 0 are Network address Host bits set to 1 are Broadcast address 127 network numbers reserved for loopback 127.0.0.1 is ‘my NIC’ 192.168.x.x & 172.16-31.x.x & 10.x.x.x are non-routable private addresses – for NAT and private networks only defined in RFC 1918 27/04/2017 ITCN

Exercise 2 Write the network & broadcast number 12.123.14.235 /24 12.123.14.235 /24 177.177.177.177 /16 220.17.124.14 /24 277.14.13.86 /33 A 27/04/2017 ITCN

Host numbers on networks Original assignment of classes wasted many IP addresses ( 3 bears problem ) Class A for governments with 224 hosts Class B can have 65 534 hosts more than most companies require Class C can have 254 hosts less than most companies require 27/04/2017 ITCN

Subnetworking Solution is to subdivide networks Take a network number Divide it into smaller networks These are called subnetworks (subnets) 27/04/2017 ITCN

Subnetting Take a /24 (class C) address 24 network bits & 8 host bits Company may need different departments We can borrow bits from host field to augment network field 27/04/2017 ITCN

Subnetting /24 (Class C) address e.g. 198.234.125.0 11000110.11101010.01111101.00000000 Write network number 10111100.11101010.00000000.00000000 Borrow 4 bits from host field 4 bits remain for host identity 27/04/2017 ITCN

Subnetted /24 (Class C) How many hosts does this allow on each subnet ? How many subnetworks can we use ? First and last subnet numbers are reserved First and last host numbers are reserved A 27/04/2017 ITCN

More Subnetting Can borrow 2 host bits minimum Maximum host bits that can be borrowed is all but the rightmost 2 bits of the IP address Why is this so ? 27/04/2017 ITCN

Calculating subnet mask 1. Express the subnetwork IP address in binary form 2. Replace the network and subnet portion of the address with all 1s 3. Replace the host portion of the address with all 0s 4. Now convert the binary expression back to dotted-decimal notation 27/04/2017 ITCN

Example of subnet mask Calculate the subnet mask for the subnetted class C address 199.177.166.34 that has borrowed 3 bits from the host field 11000111.10110001.10100110.00100010 27 bits for network identity - substitute these 27 bits for 1s Change remaining 5 host bits to 0s Convert to dotted decimal 255.255.255.224 27/04/2017 ITCN

Exercise 3 Calculate subnet masks for class B that has borrowed 4 bits from host field 155.233.2.13 Calculate subnet masks for class C that has borrowed 5 bits from host field 200.123.23.3 A 27/04/2017 ITCN

Use of Subnet Mask Mask is ANDed with IP address of host This gives network identity Can now be used to route the message 27/04/2017 ITCN

Example What is the network number for the IP address 199.177.166.34 given that the subnet mask is 255.255.255.224 Convert to binary AND both numbers together Result is network identity A 27/04/2017 ITCN

Exercise 3 IP network address 213.72.83.0. You require at least 17 subnets. Calculate 1. The required subnet mask 2. The number of total subnets 3. The number of available host IP per subnet 4. The first four subnet addresses 5. The valid range of host addresses for the first four subnets 6. The Broadcast address for each subnet 27/04/2017 ITCN

Conclusion 3 Classes available, A, B, C Strictly not used any longer but well understood Flexibility gained by subnetting Subnets concealed from outside networks using subnet masks Masks are ANDed with IP address of host to discover network identity 27/04/2017 ITCN

Hosts on class A There are a possible 224 = 16 777 216 combinations available for the last 24 bits Subtract the two combinations that cannot be used as host addresses A class A address can have 16 777 216 - 2 = 16 777 214 different hosts 27/04/2017 ITCN

Hosts on class B There are a possible 216 = 65 536 combinations available for the last 16 bits Subtract the two combinations that cannot be used as host addresses A class B address can have 65 536 - 2 = 65 534 different hosts 27/04/2017 ITCN

Hosts on class C There are a possible 28 = 256 combinations available for the last 8 bits Subtract the two combinations that cannot be used as host addresses A class B address can have 256 - 2 = 254 different hosts 27/04/2017 ITCN

Network Address 10101111.00001101. 00000000.00000000 175.13.0.0 It means ‘ this network ’ 27/04/2017 ITCN

Broadcast number 10101111.00001101. 11111111.11111111 175.13.255.255 Reaches all hosts on a network Will not reach other hosts on different network 27/04/2017 ITCN

Classes & numbers 12.123.14.0 12.123.14.255 (looks like Class A) 177.177.0.0 177.177.255.255 (Class B) 220.17.124.0 220.17.124.255 (Class C) 277.14.13.86 Illegal number And illegal subnet mask 27/04/2017 ITCN

Subnetted Class C 24 = 16 -2 = 14 hosts 24 = 16 subnetworks Note that 14 hosts x 16 subnets is less than 256 Why is this ? 27/04/2017 ITCN

Subnet masks 11111111.11111111.11110000.00000000 255.255.240.0 11111111.11111111.11111111.11111000 255.255.255.248 27/04/2017 ITCN

Network mask discovery Convert to dotted decimal 199.177.166.32 How many hosts on this subnet ? 27/04/2017 ITCN

Exercise 3 Need 17 subnets. Calculate bits required for 17 subnets = 5 Leaves 3 bits for host identity on subnets When calculating the first four subnet addresses, you will begin with: 00000000 = 0 00001000 = 8 00010000 = 16 00011000 = 24 27/04/2017 ITCN

Exercise 3 contd 3 bits for the host leaves 23 - 2 = 6 host addresses per subnet Valid host range 213.72.83.1 213.72.83.2 213.72.83.3 213.72.83.4 213.72.83.5 213.72.83.6 27/04/2017 ITCN