Wednesday, 5 September 2018

SMTP (Simple Mail Transfer Protocol) Commads, Responses and Mail transfer Phases.

SMTP (Simple Mail Transfer Protocol) uses some commands and the responses to transfer message between an MTA Client and MTA Server.


1.    Hello:-

This command used by the client to identify itself.
For example - user sends own email with HELO.
HELO: (Client mail)

2.    MAIL From:-

This command used by the client to identify the sender of the message.
Example: MAIL From: (Sender mail)

3.    RCPT To:-

This command used for identify the receiver.
Example: RCPT To:

4.    DATA:-

This command is used for send the actual data or actual message.

5.    Quit:-

The Quit command used for terminates the message.

6.    RSET:-

This Command aborts the current mail transaction and the connection is reset.

7.    VRFY:-

This command used for verify the receiver address.
Example: VRFY:

8.    HELP:-

This command asks the receiver to send information about the command sentence argument.
Example: HELP: mail

Example of Responses:-

1.    Positive Condition Reply:-

211- System Status
214- Help
220- Service Ready
221- Service Closing

2.    Transient Negative Completion Reply:-

421- Service NA
450- Mail NA
451- Command Aborted

3.    Permanent Negative Completion Reply:-

500- Syntax Error
554- Transaction Failed

Mail transfer Phases:-

There are three types of phases.

1. Connection Establishment
2. Message Transfer
3. Connection Termination

Connection Establishment:-

Firstly MTA Server send message with code "220: Service Ready" to MTA Client. It means the service is ready and server ready for receiving mail. Then MTA Client reply with HELO message and send email address for identification ( Then MTA server replies with code "250: OK" (Positive Reply).
So the connection established.

Message Transfer:-

In message transfer, client sends “Mail from:” commands to server. Then server replies with code "250: OK". Then client sends other thing (like receiver mail and other) and server reply with code "250: OK". 
Now the client sends the actual data (like- complete header and body transformation) and server replies with "250: OK".
So it’s the process of message transfer.

Connection Termination:-

The client will sends a Quit message to server. If the server is ready for Quit connection then server replies with code "221: service closed".
So these are the Mail Transfer Phases.

What is SMTP?

SMTP (Simple Mail Transfer Protocol):-

The SMTP stands for Simple Mail Transfer protocol. It is an internet standard for electronic mail (E-Mail) transmission. Simple Mail Transfer Protocol published on 1982 and defined in RFC 821. In 2008 SMTP updated version "Extend SMTP" launch and defined in RFC 5321. The Extend SMTP we are use today. SMTP uses TCP port number 25. 

All use SMTP to send or receive mails (like- Gmail, Yahoo Mail and Outlook).For sending electronic mail (E-Mail), system requires MTA (Message Transfer Agent) client and a server for receives mail on the internet. The Protocol that defines the MTA Client and the server is that protocol called SMTP (Simple Mail Transfer Protocol). So the SMTP protocol defines MTA client and MTA server in the internet.

Use of SMTP:-

Suppose these are two computers, computer A and computer B. Both computer are connected internet with LAN/WAN.

When the computer A sends any mail so the Computer A use SMTP first time.

When computer A sends any mail to self-mail server then it's use SMTP.
SMTP used only between sender mail server and receiver mail server. 

SMTP not used between receiver mail server and receiver computer.

Note:- So the range of SMTP is from Computer A to computer B's mail server.

Also read SMTP (Simple Mail Transfer Protocol) Commads, Responses and Mail transfer Phases.

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Friday, 31 August 2018

What is STP (Spanning Tree Protocol) and How STP works.


The STP stands for Spanning Tree Protocol. It’s network protocol and used for prevent layer 2 (Data Link Layer) loops. By the help of Spanning Tree Protocol (STP) loop free logical technology is built in Ethernet Networks. The Spanning Tree Protocol standardized as IEEE 802.1D.  STP blocks some ports or switches with redundant links to prevent broadcast storms and ensure loop free technology.
Spanning Tree Protocol uses the Spanning Tree Algorithm (SPA) to create a topology database of the network. Spanning Tree Algorithm invented by the Radia Perlman. For prevent loops, SPA place some interfaces in forwarding states and some interfaces in blocking states.
In Spanning Tree Protocol all switches select a root bridge. All working interfaces on the root bridge are placed in forwarding state. The Switch whose have lowest switch id will become the root bridge. All other switches called “nonroot bridges”. They nonroot bridges decide the best path to get to the root bridge.

How STP works:-

According to diagram, SW2 sends a broadcast frame to SW1 and SW3. Both switches receive the frame and forward the frame every port. SW1 sends the frame to SW3 and SW3 receives the frame and sends the frame back to SW2. Now the SW2 sends frame to SW1 and the again SW1 sends the frame to SW3. Now the same work is done by SW3. Now without STP after sometime the loop is creates in network.

 If the switch configure with STP then the STP block the port and prevent loop in network.

Sunday, 26 August 2018

What is DHCP (Dynamic Host Configuration Protocol) and How DHCP Works?


DHCP stands for Dynamic Host Configuration Protocol. Its network management protocol and used for automatically assign IP (Internet Protocol) address to device. By IP address, device able to communicates with each other devices on network or internet. DHCP Server also provides Subnet Mask, DNS server and Gateway. Dynamic Host Configuration Protocol mostly used in large area networks because is not possible for human being to give IP address every device. The Dynamic Host Configuration Protocol is based on “Client-Server model”. If the DHCP is not available then we needs to give IP address to device for communicate. When we don’t assign any IP address to device then devices automatically configure itself with IP address by using APIPA (Automatic Private IP Address). APIPA provides class B range IP address and APIPA range is to 

How DHCP works:-

When client boots up so the client send a broadcast request for find DHCP Server. The broadcast request called “DISCOVER” or “DISCOVERDHCP”. If DHCP Server is available then DHCP Server receives “DISCOVER” request packets. Now DHCP Server searches the availability of IP address and temporarily reserves that address for the client. The server also configures DNS and Gateway. Then DHCP Server send “DHCPOFFER” request packet with IP address information to Client. If the client use this IP address than Client sends “DHCPREQUEST” packet to DHCP Server. Now the DHCP Server send an “ACK” for known Client using that IP address and the client get an IP address.

Monday, 6 August 2018

What is DNS and how DNS works


The DNS stands for Domain Name System. Its a network protocol and used for domain to IP mapping. DNS is naming system for computer and services and private network. The Domain Name System manages a domain directory and translates domain name into IP address. 

When we type any website domain name (such as, the DNS changes this domain name with Internet protocol.

Why we used DNS:-

DNS is not required to established a network connection but it is much more user friendly for human because we are not able to learn every website IP address Many websites are available on internet but we learn website name (Such as or By DNS we are able to connect website by domain name.

How DNS Works:-

When a user typing  in a browser, a DNS request is send to the DNS server and asking for the IP address of the domain name.   If domain IP address available in DNS directory then then DNS server reply with the IP address and the user access

Saturday, 21 July 2018

ICMP (Internet Control Message Protocol).


ICMP stands for Internet Control Message Protocol and it’s a network layer protocol (Layer3 in OSI Model). ICMP is a part of Internet Protocol and defined in RFC 792. ICMP reports error and provides information for packets processing.

ICMP used by network devices to send error message like host unreachable and request time out. ICMP commonly used in network tool such as ping utility. There are many ways to used ICMP.
Such as ICMP used for troubleshoot internet connection by network administrator and used for other utilities. The ICMP packets size is 8 byte. The first 4 byte use for format and the last 4 byte depends on code or type.

How ICMP Works:-

When a computer is turn on and wants to test server reachability in the network. It starts ping utility and sends ICMP ECHO request packets to server.  If server is reachable the server replies with ICMP ECHO reply packets. If computer receives no response from server there might problem on the network.

Sunday, 24 June 2018

What is EIGRP (Enhanced Interior Gateway Routing Protocol).


EIGRP stands for Enhanced Interior Gateway Routing Protocol. It advanced distance-vector routing protocol and also called Hybrid protocol. It uses DUAL (Diffusing Update Algorithm) for determine best the path in network. EIGRP designed by the CISCO and only runs on CISCO routers and some CISCO switches. In 2013 EIGRP convert in open standard protocol and in 2016 EIGRP defined in RFC 7868.
EIGRP is a dynamic protocol because it automatically shares route information. In same autonomous system by using EIGRP a router share own routes to others routers and every router manage a routing table. When routing table changes then EIGRP only sends only changeable updates. 

EIGRP reduce the router workload. Each router sends hello packets for know own neighbors if any router don't send hello then it’s inactive. EIGRP uses for multicast address and protocol number 88.

EIGRP Features:-

EIGRP has many useful features that make it especially useful in large area network and complex network.

EIGRP supports VLSM and CIDR. CIDR stands for Classless Inter-Domain Routing and VLSM stands for Variable Length Subnet Mask.

EIGRP supports load balancing and uses different authentication password at different times. It's uses MD 5 and simple text authentication.
EIGRP Communication via Reliable Transport Protocol (RTP) and it not do broadcast.

EIGRP don’t sends entire routing table. It sends only changes if route changed. The default AD (Administrative Distance) value is 90.

By default EIGRP allows 100 hop counts but we can change and set hop counts 1 to 255. EIGRP maximum allows 255 hop counts.