Computer communication

Networks

ProtocolsProtocol Meaning Application

DNS Domain Name System (Server) Translates domain names such as ocr.org.uk into IP Addresses

TLS/SSL Transport Layer Security /Secure Sockets Layer

Cryptographic protocols designed for secure communications

FTP File transfer Protocol For copy files from one host to another

HTTP Hypertext Transfer Protocol For distributing hypermedia files – essentially web pages

IMAP Internet Message Access Protocol

One method for accessing emails.

POP3 Post Office Protocol (V3) Another method for accessing emails – used by webmail services

Telnet Allows bi-directional text communications on a network

Addressing

IP Addresses• An Internet Protocol (IP) address is a unique reference

number that is allocated to devices on a computer network that uses the Internet Protocol.

• It is used to identify the device that sends data over the network AND the device that is to receive the data.

• An IP address can be private, (for use on a LAN) or public (for use on the Internet or another WAN).

• The IP address allocated to a device on a network can be static (assigned by a system administrator) or dynamic, (assigned by another device on the network and different each time the connection is made).

Addressing

IP Addresses• Using IPv4, IP addresses are stored as 32-bit numbers, although for our

convenience they are usually displayed as a series of 4 decimal numbers, each one representing 8 bits of the original binary address.– Binary version (32-bit):

11001001 0100000 010110100 11111111 – Decimal version:

201.64.182.255

• The new IPv6 IP address system uses 128-bit numbers but for our convenience they are usually displayed as a series of 8 hexadecimal numbers, each one representing 16 bits of the original binary address.– Binary version (128-bit):

00100001110110100000000011010011000000000000000000101111001110110000001010101010000000001111111111111110001010001001110001011010

– Hexadecimal version:21DA:00D3:0000:2F3B:02AA:00FF:FE28:9C5A 

Addressing

MAC Addresses• In computer networking, a Media Access Control

address (MAC address) is a unique 48-bit number assigned to a network interface card (NIC) to identify it on a LAN.

• Because they are so long, MAC addresses are usually displayed in hexadecimal, for example 00-0C-E7-5D-A8-AD.

• MAC addresses are limited to being used on a LAN but IP addresses can be used on multiple types of networks including the Internet.

Packets

PacketsModern computer networks, including the Internet, carry data by

breaking it down into a series of distinct units called packets, rather than sending it as a continuous stream of data.

A typical packet might contain 1,000 to 1,500 bytes and has two parts:• Payload

– This is the user data that is to be delivered and is located between the packet headers and footers.

• Header– This provides the data that the network needs to deliver the payload, for

example the source and destination addresses.– It will also have error checking data such as checksums.– It will also contain the packet sequence number so the packet can be

reassembled in the correct order.

Packets

Packets

Source Address

Destination Address

Packet Sequence Number

Data Checksum

Packets

The data is split intoa number of smallerpackets that may takedifferent routes toreach the destination.