local telephone directory assistance service as special casethe machine name localhostand its equivalent ip address always mean the same local machinethis allows us to refer to servers running locally on the same computer as its clients port numbers port number is an agreed-upon numeric identifier for given conversation because computers on the net support variety of servicesport numbers are used to name particular conversation on given machine for two machines to talk over the netboth must associate sockets with the same machine name and port number when initiating network connections as we'll seeinternet protocols such as email and the web have standard reserved port numbers for their connectionsso clients can request service regardless of the machine providing it port number for exampleusually provides web pages on any web server machine the combination of machine name and port number uniquely identifies every dialog on the net for instancean isp' computer may provide many kinds of services for customers--web pagestelnetftp transfersemailand so on each service on the machine is assigned unique port number to which requests may be sent to get web pages from web serverprograms need to specify both the web server' internet protocol (ipor domain name and the port number on which the server listens for web page requests if this sounds bit strangeit may help to think of it in old-fashioned terms to have telephone conversation with someone within companyfor exampleyou usually need to dial both the company' phone number and the extension of the person you want to reach if you don' know the company' numberyou can probably find it by looking up the company' name in phone book it' almost the same on the net--machine names identify collection of services (like company)port numbers identify an individual service within particular machine (like an extension)and domain names are mapped to ip numbers by domain name servers (like phone bookwhen programs use sockets to communicate in specialized ways with another machine (or with other processes on the same machine)they need to avoid using port number reserved by standard protocol--numbers in the range of to --but we first need to discuss protocols to understand why the protocol layer although sockets form the backbone of the internetmuch of the activity that happens on the net is programmed with protocols,which are higher-level message models that some books also use the term protocol to refer to lower-level transport schemes such as tcp in this bookwe use protocol to refer to higher-level structures built on top of socketssee networking text if you are curious about what happens at lower levels network scripting