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the user, who is given a higher set of commands to use.
These higher-level commands are not standardized so they may vary from one implementation of FTP to another. For some operating systems,
not all of these commands make equal sense, such as "Change Directory," or may have different meanings. Therefore the specific user
interface that the user sees will probably differ.
This file describes a generic implementation of the standard TCP/IP application protocols. Users must consult local documentation for specifics
at their sites.
Names and Addresses In A Network
In DOD Protocol Suite, each network is given a unique identifying number. This number is assigned by a central authority, namely the Network
Information Center run by SRI, abbreviated as SRI-NIC, in order to prevent more than one network from having the same network number. For
example, the ARPAnet has network number 10 while MIDnet has a longer number, namely 128.242. Each host in a network has a unique
identification so other hosts can specify them unambiguously. Host numbers are usually assigned by the organization that manages the
network, rather than one central authority. Host numbers do not need to be unique throughout the whole Internet but two hosts on the same
network need to have unique host numbers.
The combination of the network number and the host number is called the IP address of the host and is specified as a 32-bit binary number. All
IP addresses in the Internet are expressible as 32-bit numbers, although they are often written in dotted decimal notation. Dotted decimal
notation breaks the 32-bit number into four eight-bit parts or octets and each octet is specified as a decimal number. For example, 00000001 is
the binary octet that specifies the decimal number 1, while 11000000 specifies 192. Dotted decimal notation makes IP addresses much easier to
read and remember.
Computers in the Internet are also identified by hostnames, which are strings of characters, such as "phrackvax." However, IP packets must
specify the 32-bit IP address instead of the hostname so some way to translating hostnames to IP addresses must exist.
One way is to have a table of hostnames and their corresponding IP addresses, called a hosttable. Nearly every TCP/IP implementation has
such a hosttable, although the weaknesses of this method are forcing a shift to a new scheme called the domain name system. In UNIX
systems, the hosttable is often called "/etc/hosts." You can usually read this file and find out what the IP addresses of various hosts are.
Other systems may call this file by a different name and make it unavailable for public viewing.
Users of computers are generally given accounts to which all charges for computer use are billed. Even if computer time is free at an
installation, accounts are used to distinguish between the users and enforce file protections. The generic term "username" will be used in this
file to refer to the name by which the computer account is accessed.
In the early days of the ARPAnet which was the first network to use the TCP/IP protocols, computer users were identified by their username,
followed by a commercial "at" sign (@), followed by the hostname on which the account existed. Networks were not given names, per se,
although the IP address specified a network number.
For example, "knight@phrackvax" referred to user "knight" on host "phrackvax." This did not specify which network "phrackvax" was on,
although that information could be obtained by examining the hosttable and the IP address for "phrackvax." (However, "phrackvax" is a
fictitious hostname used for this presentation.)
As time went on, every computer on the network had to have an entry in its hosttable for every other computer on the network. When several
networks linked together to form the Internet, the problem of maintaining this central hosttable got out of hand. Therefore, the domain name
scheme was introduced to split up the hosttable and make it smaller and easier to maintain.
In the new domain name scheme, users are still identified by their usernames, but hosts are now identified by their hostname and any and all
domains of which they are a part. For example, the address "KNIGHT@UMCVMB.MISSOURI.EDU" specifies username "KNIGHT" on host
"UMCVMB". However, host "UMCVMB" is a part of the domain "MISSOURI" " which is in turn part of the domain "EDU". There are other domains
in "EDU", although only one is named "MISSOURI". In the domain "MISSOURI", there is only one host named "UMCVMB".
However, other domains in "EDU" could theoretically have hosts named "UMCVMB" (although I would say that this is rather unlikely in this
example). Thus the combination of hostname and all its domains makes it unique. The method of translating such names into IP addresses is
no longer as straightforward as looking up the hostname in a table. Several protocols and specialized network software called nameservers
and resolvers implement the domain name scheme.
Not all TCP/IP implementations support domain names because it is rather new. In those cases, the local hosttable provides the only way to
translate hostnames to IP addresses. The system manager of that computer will have to put an entry into the hosttable for every host that [ Pobierz całość w formacie PDF ]