Tuesday, July 8, 2008

IP: Internet Protocol

The physical layer


Networks are built in layers (e.g. the OSI stack)
Each layer is independent of the others and they are interchangeable.
Ethernet resides on the OSI Datalink layer.
IP resides on the OSI Network layer.
TCP resides on the OSI Transport layer
Other protocols that can inter-operate with IP on the Datalink layer are: Token Rink, PPP, SLIP, X.25 Frame Relay etc
Each Ethernet network card has a unique address – physical address or Media Access Control (MAC) address.
An Ethernet frame (packet) consists of a header and a payload (data).
The header contains the source address, the destination address and control information.
The payload can contain data for any protocol (e.g. IP).
If the payload contains IP data, then the MAC address is mapped to the IP address using the Address Resolution Protocol (ARP).
The source (sending) host needs to determine the MAC address for a given physical address. It broadcasts an ARP "Who has address xyz" request. XYZ responds directly with "xyz (IP) = abc (MAC)". In this way the Ethernet packet can be built and physically put on the wire.
Key concepts

ARP: Address Resolution Protocol

MAC: Media Access Control

OSI: Open Systems Interconnection

IP: Internet Protocol

PPP: Point to Point Protocol

SLIP: Serial Line Internet Protocol

Important commands

ARP (Microsoft & UNIX)

Use arp –a to display all the MAC addresses a host has in its ARP tables.

Use arp –d [address] to delete a MAC address from the table.

ipconfig (Windows NT / Windows 98)

Use ipconfig /all to see the complete IP configuration and the MAC address for any device.

winipcfg (Windows 95)

Use winipcfg to see the complete IP configuration and the MAC address for any device.

ifconfig (UNIX)

Use ifconfig -au to see the complete IP configuration and the MAC address for all devices on the host.

Tips and tricks

If two hosts are in the same IP subnet then they MUST have MAC addresses for each other in their ARP tables.
The ARP table should immediately be updated with the new address when pinging another host on the same subnet.


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