IP

IP: (Internet Protocol)

What is IP?

IP datagram

IP addressing

IP address classes

Subnet mask

IP routing

What is IP?

IP is a protocol that provides a unreliable, connectionless transmission of data passed to it from the upper layer protocols such as TCP, UDP, ICMP, via an IP datagram.

Unreliable - ie: there is no guarrantee that an IP datagram will get to its destination. If something goes wrong then IP discards the datagram and tries to send a ICMP message back to the source.
Connectionless - ie: each datagram is handled individually therefore it is possible for datagrams to get delivered out of order.
Each IP datagram is sent individually.
It is up to the upper levels to provide any required reliability, and requests to resend.

Format for an IP datagram

4-bit version	4-bit header length	8-bit type of service	16-bit total length (in bytes)
16-bit identifier			3-bit flag	13-bit fragment offset
8-bit time to live (TTL)		8-bit protocol	16-bit header checksum
32-bit source IP address
32-bit destination IP address
options if any
data

Version (4-bits) - holds the code for the current protocol.
Header length (4-bits) - is the number of 32 bit words in the header, limited to 60 bytes.
Type of Service (TOS) - consists of a 3-bit precedence field, a 4-bit Type of service field (TOS) and an unused bit that must be '0'.

Recommended values for type of service field.

Application	Minimize delay	Maximize throughput	Maximize reliability	Minimize monetary cost	Unused	Hex value
Telenet/Rlogin	1	0	0	0	0	0x10
FTP control FTP data FTP bulk data	1 0 0	0 1 1	0 0 0	0 0 0	0 0 0	0x10 0x08 0x08
TFTP	1	0	0	0	0	0x10
SMTP command SMTP data	1 0	0 1	0 0	0 0	0 0	0x10 0x08
DNS UDP query DNS TCP query DNS zero transfer	1 0 0	0 0 1	0 0 0	0 0 0	0 0 0	0x10 0x00 0x08
ICMP error ICMP query	0 0	0 0	0 0	0 0	0 0	0x00 0x00
any IGP	0	0	1	0	0	0x04
SNMP	0	0	1	0	0	0x04
BOOTP	0	0	0	0	0	0x00
NNTP	0	0	0	1	0	0x02

IP Addressing

Each host in the internetwork has a unique 32 bit IP address.
This 32 bit binary number is usually presented as a 4 part decimal number delimited by a full stop (".").
decimal - 255.255.255.255

IP Address Classes

These addresses have been divided into classes:

class	bbb	bbbbb	bbbbbbbb	bbbbbbbb	bbbbbbbb
A	000	net	host	host	host
B	100	net	net	host	host
C	110	net	net	net	host
D	1110		multicast address
E	11110		reserved for future use

Subnet Mask

The subnet mask is used to determine which part of the 32 bit address represents the network and which part represents the host.

The default subnet mask for Class A, Class B, and Class C.

Class	Default subnet mask
A	255.0.0.0
B	255.255.0.0
C	255.255.255.0

IP Routing

IP routing is achieved by first searches a routing table in memory each time a datagram is recieved to send.
Each entry contains:

The IP address of the destination.
The IP address of the next hop router.
Flags.
Specification of which network interface the datagram should be passed to for transmission.