Networking in UNIX

A brief review and frequently used commands

1.Overview of Network Concept

• A network is an interconnected system of cooperating computers.

• UNIX had a huge explosion of network applications in early 1990’s.

• Example: Clients-server systems

Common Network Terminology

• LAN

• WAN

• INTERNET

• WWW

Different Topologies

• BUS

• RING

• STAR

Ethernet

• First developed by Xerox Corporation.

• It works like this:

• 1. Each computer contains an Ethernet card, and each card has a unique Ethernet address.

• 2. Every computer’s Ethernet card is connected to the same single piece wire (Ethernet cable).

Ethernet…

• 3. A computer broadcasts its message onto the Ethernet a particular Ethernet address along with Ethernet header and trailer information. Only that computer Ethernet card has the same address accepts the message.

• 4. In case of collision-two or more computers broadcast to the Ethernet at the same time, they will wait a random period of time and then try again.

Ethernet…

• .

Ethernet Cable

Bridges

• Because there is a length limit on how long an Ethernet cable can be, we need some special devices to connect different Ethernet network together when the limit is exceeded.

• Ethernet Standard: IEEE 802.3

• See document: TechFest on my web site

Cable Bus Length Limit

• See Document: Cable Buses on my web site

Diagram• .

Routers

• Routers are introduced to connect multi-LANs together in a more efficient way.

• Example: Connect four LAN together.

•

Gateways

• To connect several LAN into a single large wide area network (WAN), we need several high-capacity routers, which we call gateways to realize this goal.

• Example: Each corporation (a University can be considered as a corporation) tie their LAN through the nearest gateway.

Protocols

• TCP/IP

• Transmission Control and Internet Protocols

The Five-Layer TCP/IP Internet Protocol Hierarchy

Physical Layer

• Protocols govern the exchange of binary digits across a physical communication channel

• Goal: create a “bit pipe” between two computers

Data Link Layer

• Protocols carry out

– Error handling

– Framing

• Creates an error-free “message pipe”

• Composed of two services

– Layer 2a: medium access control

– Layer 2b: logical link control

Data Link Layer (continued)

• Medium access control protocols

– Determine how to arbitrate ownership of a shared line when multiple nodes want to send at the same time

• Logical link control protocols

– Ensure that a message traveling across a channel from source to destination arrives correctly

Network Layer

• Delivers a message from the site where it was created to its ultimate destination

• Critical responsibilities

– Creating a universal addressing scheme for all network nodes

– Delivering messages between any two nodes in the network

Network Layer (continued)

• Provides a true “network delivery service”

– Messages are delivered between any two nodes in the network, regardless of where they are located

• IP (Internet Protocol) layer

– Network layer in the Internet

Transport Layer

• Provides a high-quality, error-free, order preserving end-to-end delivery service

• TCP (Transport Control Protocol)

– Primary transport protocol on the Internet

– Requires the source and destination programs to initially establish a connection

Application Layer

• Implements the end-user services provided by a network

• There are many application protocols, including:

– HTTP

– SMTP

– POP3

– IMAP

– FTP

Some Popular Application Protocols on the Internet

Application Layer (continued)

• Uniform Resource Locator (URL)

– A symbolic string that identifies a Web page

– Form

protocol://host address/page

– The most common Web page format is hypertext information

• Accessed using the HTTP protocol

Internet Address

• An IP address is a 32-bits value that is written as four dot-separated numbers (4 dot-separated bytes) from 0-255

• Newer IPv6 will define 128-bit IP addresses.

UNIX Utilities on Networking

• 1. users

Who And W

• who – displays every user on your local host.

• w – similar to who and it gives you a little more information on each user.

hostname

• hostname – displays the name of your local host

finger

• finger [userid] displays information about a list of users with more information.

• Example: finger zhang

• Pay attention about the extra information it displays.

Write, talk, walk, and mail

• write – sending a line a time

• You already saw talk and walk briefly before.

• mail is an old command we used a lot even before the WWW

The mesg command

• If one does not want be bothered with the public information (it almost like the pop-ups in the Internet), one can disable them by set the mesg n command.

• Example: mesg n or mesg y

ftp command