Network Communication at a high level

Network communication, Layers

OSI LayercommunicationIPv4 Addresses

OSI (Open Systems Interconnection)

Layer Transport mechanism Device Protocols
7 Application SSH, HTTP, HTTPS
6 Presentation
5 Session
4 Transport Segments TCP, UDP
3 Network Packets Router IP, ICMP
2 Datalink Frames Switch Ethernet, ARP
1 Physical Bits Hub

Physical layer

  • Actual medium of data transfer.
    • Copper cables (uses Electric signals); Optical Fiber cables (uses Light); Wireless (Uses Radio Frequency)
  • Hubs are Layer 1 devices. They’re nothing more than a Repeater. A Hub does not have any intelligence, It simply forwards the traffic it receives across all the connected devices.

Datalink layer

  • Switch is Layer 2 device. It can understand Data Link Frames. A switch maintains MAC addresses and the ports of all devices connected to it.
  • Each Frame encapsulates the upper layer data unit, which is IP Packet.
  • Every frame also has the source and destination MAC addresses. These are used to route frames in the same network.

Network layer

  • Router is Layer 3 device. It can understand IP packets. It is used to forward IP packets from one network to another. It maintains a route table that tells the next destination for the IP packet to which the packet needs to be routed.
  • Each packet encapsulates the upper layer data unit. It could be a TCP Segment or UDP Datagram.
  • A packet also has source and destination IP addresses. The IP addresses belong to the network or different networks.

Transport layer

Application layer

IP addresses
Class A 0.0.0.0 to 127.255.255.255 128 Networks (From 0 to 127). Default netmask /8. Each network with 2 power 24 IP addresses.
Class B 128.0.0.0 to 191.255.255.255 Network - 128.0 to 191.255. Default netmask /16. Each network with 2 power 16 hosts.
Class C 192.0.0.0 to 223.255.255.255 Network - 192.0.0 to 223.255.255. Default netmask /24. Each network with 254 hosts.
Class D Used for Multicast
Class E Reserved

Private IPv4 Address Space

  • 10.0.0.0 to 10.255.255.255
    • 1 Class A network
  • 172.16.0.0 to 172.31.255.255
    • 16 Class B networks
  • 192.168.0.0 to 192.168.255.255
    • 255 Class C networks

Note that, for a given IP range for a network, if we use Default subnet mask /8 or /16 or / 24, it is called Classful network. Otherwise it is called Classless network. For example, networks starting with 10.X.X.X are Class A networks. If we use 10.0.0.0/8 it is called Classful network as /8 is the default mask for Class A. However, if we use 10.0.0.0/16 where network is 10.16.0.0 it is called Classless network as we are using /16.

Subnetting

Let’s say I have a network 10.0.0.0/24.

  • /24 means, out of 32 bits of the IP address, first 24 bits do not change for each host in this network. For each host, first 24 bits are the same. Another way to represent this is using a Subnet mask . 255.255.255.0 is the subnet mask to represent /24.
  • This means that the network ID is 10.0.0.0 and the network can have 255 addresses (With 24 bits taken by Network ID, we are left with 8 bits. With 8 bits, we can have maximum 255 unique values). But, the last IP is used for broadcasting.
    • 10.0.0.0 is the network ID. It cannot be used to assign to a host.
    • 10.0.0.255 is the Broadcast IP. It cannot be used either to assign to a host.
  • At this point, we have 254 addresses that can be assigned to devices in the network.

How to split this network into 4 sub networks?

  • To identify each subnet, we need subnet ID. However, the first 24 bits are already taken by the original network. so, we have to borrow bits from the host bits to create new subnets. For 4 subnets, we need 2 bits. So, out of 8 bits, 2 bits will be used for Subnet ID, and 6 bits will be used for Hosts in the subnet.
  • The Network IDs would now be /26 rather than /24.

The four subnets look like this:

10.0.0.0 10.0.0.64 10.0.0.128 10.0.0.192
10.0.0.1 10.0.0.65 10.0.0.129 10.0.0.193
10.0.0.2 10.0.0.66 10.0.0.130 10.0.0.194
10.0.0.3 10.0.0.67 10.0.0.131 10.0.0.195
10.0.0.4 10.0.0.68 10.0.0.132 10.0.0.196
10.0.0.5 10.0.0.69 10.0.0.133 10.0.0.197
10.0.0.6 10.0.0.70 10.0.0.134 10.0.0.198
10.0.0.7 10.0.0.71 10.0.0.135 10.0.0.199
10.0.0.8 10.0.0.72 10.0.0.136 10.0.0.200
10.0.0.9 10.0.0.73 10.0.0.137 10.0.0.201
10.0.0.10 10.0.0.74 10.0.0.138 10.0.0.202
10.0.0.11 10.0.0.75 10.0.0.139 10.0.0.203
10.0.0.12 10.0.0.76 10.0.0.140 10.0.0.204
10.0.0.13 10.0.0.77 10.0.0.141 10.0.0.205
10.0.0.14 10.0.0.78 10.0.0.142 10.0.0.206
10.0.0.15 10.0.0.79 10.0.0.143 10.0.0.207
10.0.0.16 10.0.0.80 10.0.0.144 10.0.0.208
10.0.0.17 10.0.0.81 10.0.0.145 10.0.0.209
10.0.0.18 10.0.0.82 10.0.0.146 10.0.0.210
10.0.0.19 10.0.0.83 10.0.0.147 10.0.0.211
10.0.0.20 10.0.0.84 10.0.0.148 10.0.0.212
10.0.0.21 10.0.0.85 10.0.0.149 10.0.0.213
10.0.0.22 10.0.0.86 10.0.0.150 10.0.0.214
10.0.0.23 10.0.0.87 10.0.0.151 10.0.0.215
10.0.0.24 10.0.0.88 10.0.0.152 10.0.0.216
10.0.0.25 10.0.0.89 10.0.0.153 10.0.0.217
10.0.0.26 10.0.0.90 10.0.0.154 10.0.0.218
10.0.0.27 10.0.0.91 10.0.0.155 10.0.0.219
10.0.0.28 10.0.0.92 10.0.0.156 10.0.0.220
10.0.0.29 10.0.0.93 10.0.0.157 10.0.0.221
10.0.0.30 10.0.0.94 10.0.0.158 10.0.0.222
10.0.0.31 10.0.0.95 10.0.0.159 10.0.0.223
10.0.0.32 10.0.0.96 10.0.0.160 10.0.0.224
10.0.0.33 10.0.0.97 10.0.0.161 10.0.0.225
10.0.0.34 10.0.0.98 10.0.0.162 10.0.0.226
10.0.0.35 10.0.0.99 10.0.0.163 10.0.0.227
10.0.0.36 10.0.0.100 10.0.0.164 10.0.0.228
10.0.0.37 10.0.0.101 10.0.0.165 10.0.0.229
10.0.0.38 10.0.0.102 10.0.0.166 10.0.0.230
10.0.0.39 10.0.0.103 10.0.0.167 10.0.0.231
10.0.0.40 10.0.0.104 10.0.0.168 10.0.0.232
10.0.0.41 10.0.0.105 10.0.0.169 10.0.0.233
10.0.0.42 10.0.0.106 10.0.0.170 10.0.0.234
10.0.0.43 10.0.0.107 10.0.0.171 10.0.0.235
10.0.0.44 10.0.0.108 10.0.0.172 10.0.0.236
10.0.0.45 10.0.0.109 10.0.0.173 10.0.0.237
10.0.0.46 10.0.0.110 10.0.0.174 10.0.0.238
10.0.0.47 10.0.0.111 10.0.0.175 10.0.0.239
10.0.0.48 10.0.0.112 10.0.0.176 10.0.0.240
10.0.0.49 10.0.0.113 10.0.0.177 10.0.0.241
10.0.0.50 10.0.0.114 10.0.0.178 10.0.0.242
10.0.0.51 10.0.0.115 10.0.0.179 10.0.0.243
10.0.0.52 10.0.0.116 10.0.0.180 10.0.0.244
10.0.0.53 10.0.0.117 10.0.0.181 10.0.0.245
10.0.0.54 10.0.0.118 10.0.0.182 10.0.0.246
10.0.0.55 10.0.0.119 10.0.0.183 10.0.0.247
10.0.0.56 10.0.0.120 10.0.0.184 10.0.0.248
10.0.0.57 10.0.0.121 10.0.0.185 10.0.0.249
10.0.0.58 10.0.0.122 10.0.0.186 10.0.0.250
10.0.0.59 10.0.0.123 10.0.0.187 10.0.0.251
10.0.0.60 10.0.0.124 10.0.0.188 10.0.0.252
10.0.0.61 10.0.0.125 10.0.0.189 10.0.0.253
10.0.0.62 10.0.0.126 10.0.0.190 10.0.0.254
10.0.0.63 10.0.0.127 10.0.0.191 10.0.0.255
  • The last IP address is each subnet is called Brocdcast IP address. here, 10.0.0.63/26, 10.0.0.127/26, 10.0.0.191/26, 10.0.0.255/26 are the broadcast IP address respectively.

How two devices in the same network communicate?

  • host 2 is running a web server on port 80.
  • A user on host 1 opened a web browser and typed http://10.0.0.2. Web browser falls under Application layer.
    • The transport layer creates a segment with a random port (say 12567 as the source port and 80 as the target port)
    • The network layer creates a packet by encapsulating the segment. The packet has source IP as 10.0.0.1 and destination IP as 10.0.0.2
    • The Datalink layer takes the packet and creates a Frame. However, It does not know the MAC address of the target machine. Here, host 1 needs to identify whether host 2 is on the same network or different network. If on the same network, traffic does not have to leave the network. In this case, as both are on same network, host 1 realizes that it needs the MAC address of host 2.
  • host 1 creates a Broadcast frame with Destination MAC address as 10.0.0.255. When the switch sees the broadcast address, it broadcasts the frame to all hosts in the network. host 2 responds with its MAC address which is then received by host 1. This is Address Resolution Protocol.
  • host 1 creates a Frame by encapsulating the packet with source MAC as ea:e9:1c:b7:20:71 and target MAC address as ea:e9:1c:b7:20:72 and forwards it to the switch.
  • The switch knows the MAC addresses and the ports of each device connected to it using the MAC address table. It identifies that host 2 is the target and forwards it.