2.5 Network Service

Network Service

• Network services are the foundation of a networked computing environment.

• Generally network services are installed on one or more servers to provide shared resources to client computers.

• Network services are configured on corporate LAN’s to ensure security and user friendly operation.

• They help the LAN run smoothly and efficiently.

Domain Name Service (DNS)

• The Domain Name System (DNS) is a hierarchical naming system for computers, services, or any resource connected to the Internet or a private network.

• It associates various information with domain names assigned to each of the participants.

• Most importantly, it translates domain names meaningful to humans into the numerical (binary) identifiers associated with networking equipment for the purpose of locating and addressing these devices worldwide.

2.5.1

Domain Name Service (DNS) continued.....

• An often-used analogy to explain the Domain Name System is that it serves as the "phone book" for the Internet by translating human-friendly computer hostnames into IP addresses.

• For example, www.example.com translates to 192.0.32.10.

• The Domain Name System makes it possible to assign domain names to groups of Internet users in a meaningful way, independent of each user's physical location.

• Because of this, World Wide Web (WWW) hyperlinks and Internet contact information can remain consistent and constant even if the current Internet routing arrangements change or the participant uses a mobile device.

Domain Name Service (DNS) continued.....

• Internet domain names are easier to remember than IP addresses such as 208.77.188.166 (IPv4) or 2001:db8::1f70:6e8 (IPv6).

• People take advantage of this when they recite meaningful URLs and e-mail addresses without having to know how the machine will actually locate them.

• The Domain Name System distributes the responsibility of assigning domain names and mapping those names to IP addresses by designating authoritative name servers for each domain.

Ping command, a browser and nslookup

• ping command– Ping is a computer network administration utility used to

test whether a particular host is reachable across an Internet Protocol (IP) network and to measure the round-trip time for packets sent from the local host to a destination computer, including the local host's own interfaces.

– Ping operates by sending Internet Control Message Protocol (ICMP) echo request packets to the target host and waits for an ICMP response.

2.5.2

Ping command, a browser and nslookup

• ping command– sample of ping command:

Ping command, a browser and nslookup

• browser– A network browser is a tool used to browse a

computer network.

– An example of this is My Network Places (or Network Neighborhood in earlier versions of Microsoft Windows).

– An actual program called Network Browser is offered in Mac OS 9.

Ping command, a browser and nslookup

• nslookup– nslookup is a computer program used in Windows

and Unix to query Domain Name System (DNS) servers to find DNS details, including IP addresses of a particular computer, MX records for a domain and the NS servers of a domain.

– The name nslookup means "name server lookup".

– The most common version of the program is included as part of the BIND package.

Ping command, a browser and nslookup• nslookup– More modern alternatives to nslookup are the "host"

and "dig" programs which also ship with BIND, all of which look up nameserver information in /etc/resolv.conf. nslookup can warn you that you're looking at cached information (which host does not do), so it remains useful for this reason.

– nslookup has the subcommands:server NAME (where NAME is the name or IP address of a

DNS server to query). set type=NAME (where NAME is the type of record to look

at). For example, set type mx will give the mail records.

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Ping command, a browser and nslookup• nslookup– Sample of nslookup:

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DNS in LAN

• Corporate LANs use network services such as DNS to give names to IP and MAC addresses and DHCP to ensure that everyone on the network has a valid IP address.

• For example, names like “nm.lan” is better than numbers like “210.121.67.18”,

2.5.3

Relationship between Web Clients and Server

• A web server is a computer program that delivers (serves) content, such as web pages, using the Hypertext Transfer Protocol (HTTP), over the World Wide Web.

• The term web server can also refer to the computer or virtual machine running the program.

• The primary function of a web server is to deliver web pages to clients.

2.5.4

Relationship between Web Clients and Server

• This means delivery of HTML documents and any additional content that may be included by a document, such as images, style sheets and JavaScripts.

• A client, commonly a web browser or web crawler, initiates communication by making a request for a specific resource using HTTP and the server responds with the content of that resource, or an error message if unable to do so

• The resource is typically a real file on the server's secondary memory, but this is not necessarily the case and depends on how the web server is implemented.

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Relationship between FTP Clients and Server

• File Transfer Protocol (FTP) is a standard network protocol used to copy a file from one host to another over a TCP/IP-based network, such as the Internet.

• FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server applications, which solves the problem of different end host configurations (i.e., Operating System, file names).

• FTP is used with user-based password authentication or with anonymous user access.

• Applications were originally interactive command-line tools with a standardized command syntax, but graphical user interfaces have been developed for all desktop operating systems in use today.

2.5.5

Relationship between FTP Clients and Server (2)

• A client makes a connection to the server on TCP port 21.

• This connection, called the control connection, remains open for the duration of the session, with a second connection, called the data connection, on port 20 opened as required to transfer file data.

• The control connection is used to send administrative data (i.e., commands, identification, passwords).

• Commands are sent by the client over the control connection in ASCII and terminated by a carriage return and line feed.

Relationship between FTP Clients and Server (3)

• The server responds on the control connection with three digit status codes in ASCII with an optional text message, for example "200" (or "200 OK.") means that the last command was successful.

• The numbers represent the code number and the optional text represent

• A file transfer in progress over the data connection can be aborted using an interrupt message sent over the control connection.

Email Protocol in client and server

• SMTP• POP3• IMAP4

2.5.6

Email Protocol in client and server• SMTP– Simple Mail Transfer Protocol (SMTP) is an

Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks.

– SMTP was first defined in RFC 821 and last updated by RFC 5321 which includes the extended SMTP (ESMTP) additions, and is the protocol in widespread use today.

– SMTP is specified for outgoing mail transport and uses TCP port 25.

Email Protocol in client and server• SMTP– While electronic mail servers and other

mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying.

– For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) or a proprietary system (such as Microsoft Exchange or Lotus Notes/Domino) to access their mail box accounts on a mail server

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Email Protocol in client and server• POP3 – The Post Office Protocol (POP) is an application-layer

Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection.

– POP and IMAP (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval.

– Virtually all modern e-mail clients and servers support both.

Email Protocol in client and server• POP3 – The POP protocol has been developed through

several versions, with version 3 (POP3) being the current standard.

– POP3 is used for most mail clients such as gmail and yahoo.

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Email Protocol in client and server• IMAP4 – The Internet Message Access Protocol (IMAP) is one

of the two most prevalent Internet standard protocols for e-mail retrieval, the other being the Post Office Protocol (POP).

– Virtually all modern e-mail clients and mail servers support both protocols as a means of transferring e-mail messages from a server