IP Addresses - IPV4 - Subnetting - IP Address Classes

Fragmentation

A packet is the constructed data to be sent over a network using a reliable network. A Datagram is the same but on an un-reliable network.

Packet
A packet is "unit" of network transmitted data. This "data" includes the information the sender wants to transmit as well as additional addressing information such as the senders IP Address. When this "data" is sent over a reliable network it is considered a packet.
Datagram
A datagram is a "unit" of network transmitted data, like a packet, but over an unreliable network.
Fragment
A fragment is a self contained packets or datagrams depending on the type of network.
Fragmentation
Since the source does not know the Path MTU, it can break the data into smaller bits of data contained in a packet or datagram. This is called Fragmentation. The "second law of thermodynamics" is about how taking a large object into smaller ones is easier then the reverse. That is added because Packet Switching Networks have to re-assemble the fragments into packets and this can be a troublesome process.
Path MTU (Path maximum Transmission Unit)
How small the packet must be to get to its destination. Basically the size of a packet for the least common denominator packet size along the path.
Transparent Fragmentation
The packet is sent to a router, if its oversized, the router breaks it into fragments. Each fragment is sent to the same exit router where it builds the packet back.
Nontransparent Fragmentation
Packets are not reconstructed - IP works with way. Each fragment is treated as the full packet at each router. The destination then reconstructs the full packet from the individual fragments by reading the packet numbers in fragmented packets. This allows to the routers to do less work.
Path MTU discovery
This process involves the Destination setting the header info in all packets to tell routers no fragmentation is allowed. If a router along the path needs fragmentation on a packet, its sends the error packet back to the source, where it re-fragments the packet down more, and re-sends the packets.

IP Address Classes

The first octet is the first 8 bits in in the address.

High order bits are....

A class defines which bits are the network ID and which are the host ID as well as the possible number of networks and the number of hosts for each network.

Class A

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
0 Network ID Host ID
  • Large sized networks
  • 126 networks - 16,777,214 hosts / network
  • high-order bit set to 0
  • following 7 are the Network ID
  • The Host ID is the remaining 3 octets (24 bits)

Class B

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
1 0 Network ID Host ID
  • Medium -> large sized networks
  • 16,384 networks - 65,534 hosts / network
  • Two high-order bits set to 1 0 respectively
  • Next 14 (end of first two octets) are the network ID.
  • Remaining 16 bits (two octets) are the host ID

Class C

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
1 1 0 Network ID Host ID
  • Smaller networks
  • 2,097,152 networks - 254 hosts / network
  • Three high-order bits set to 1 1 0 respectively
  • Next 21 (end of first three octets) are the network ID.
  • Remaining 8 bits (last octet) are the host ID
Class High Order Bit (HOB - binary) Value (in decimal) Default Subnet Mask (DSM) /n
A 0 1 - 126 255.0.0.0 /8
B 10 127 - 191 255.255.0.0 /16
C 110 192 - 223 255.255.255.0 /24
IP Address 1st Octet Value In Binary Class (HOB - binary) Value (in decimal) (DSM) /n
192.168.1.1 1 1 0 0 0 0 0 0 C 11 192 255.255.255.0 /24
67.162.79.135 0 0 1 1 1 1 1 0 A 0 67 255.0.0.0 /8
184.163.19.15 1 0 1 1 1 0 0 0 B 10 184 255.255.0.0 /16