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NTP:-NTP stands for Network Time Protocol. Network time synchronization technology was first used in 1979. NTP was designed by David L Mills in University of Delaware. NTP used for clock synchronization between computer systems or data network. NTP server that’s connects through the internet to an atomic clock. This time can then be synchronized through the network to keep all routers, switches, servers etc. receiving the same time information. NTP is one of the oldest protocols that are used in current. NTP uses UDP (User Datagram Protocol) protocol. NTP works on port number 123.
NTP synchronize all participating computer within few milliseconds of UTC (Universal Time Coordinated). UTC is a coordinated time scale and maintained by the Bureau International de Poids at Mesures (BIPM). It is also known as “Z Time” or “Zulu Time”. NTP uses the intersection algorithm, a modified version of Marzullo’s algorithm, to select accurate time servers and is designed to mitigate the effects of variable network latency. NTP can usually maintain time over the public network. 
How NTP works:-
The NTP client send request to NTP server for asks the right time. The server checks the client wrong time and send acknowledgements with the right time. Then client open this acknowledgement and set automatically set the time.


Why correct network time is important in network:-By correct time, tracking of events are allows on the network in correct order.Clock synchronization is critical for the correct interpretation of events within the syslog data.Clock synchronization is critical for digital certificates.
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In the last article we were discuss about VTP (Vlan Trunking Protocol) and how VTP works. Now in this article i will tell you about VTP (Vlan Trunking Port) Modes. Each switch can us three different VTP Modes.


VTP Modes:-

3. VTP (Vlan Trunking Port) Transparent Mode

VTP Mode Explained:-1. VTP Server Mode:-
 The VTP (Vlan Trunking port) Server Mode is the default mode for cisco switches. In this mode we can create or delete VLAN's on switches and the changes are advertised to the entire VTP doamin. In VTP Server Mode switch synchronizes VLAN configuration information with other switches and all VLAN's configurations are saved in NVRAM on the switch.

2. VTP Client Mode:- 
In VTP (Vlan Trunking Port) Client Mode switch can not change its VLAN configuration. It means we can not create or delete Vlan's on switches. In VTP Client Mode switch receive information from VTP servers and forwards VTP advertisements. The VLAN's configurations are not saved in NVRAM on the switch.  
3. VTP Transparent Mode:- In VTP (Vlan Trunking Port) Transparent Mode switch do not participate in VTP domain and do not share its Vlan database. In this mode switch also forwards received VTP advertisement. We can create or delete VLAN but the changes are not sent to other switches. All VLAN's configuration are saved in NVRAM on the switch.
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VTP (VLAN Trunking Protocol):-
VTP stands for VLAN Trunking Protocol. VLAN trunking protocol is a Cisco proprietaryprotocol and it’s used by Cisco switches to exchange VLAN information. By the help of VTP, we can synchronize VLAN information with switches inside the same VTP domain. For example we can synchronize VLAN id or VLAN name. VTP uses layer 2 frames to manage the addition and deletion and renaming of VLANs from switchesin the VTP client mode.
Suppose we have 100 switches. Without VTP, if we want to create a VLAN on each switch, we would have to manually enter commands to create the VLAN only on one switch. That switch can then propagate information about that VLAN to each switch on a network and cause other switches to create that VLAN too. If you want to delete a VLAN, we only need to delete it on one switch and the change is automatically propagated to every other switch inside the same VTP domain.
How VTP works:-
We have three switches and we have created a new VLAN on SW1. SW1 sends a VTP update to SW2 and SW2 sends own VTP update to SW3. Now all three switches have the same VLAN.

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ACL (Access Control List):-
The ACLs stands for Access Control List.  Access Control List (ACL) is a list of permission and used for filter network traffic. Access Control List (ACL) used on network devices with packet filtering capabilities like Router or Firewalls. ACL tells to network devices what types of packets accept or deny. ACL’s are applied on the interfaces to packets leaving or entering.
ACL provides basic level of security for network access and increase network performance because the traffic is limited.
How ACL's Works:-
Suppose a company who has 2000 users and 500 employees. The company server (Server1) holds some important documents. They documents are available only for company employees and users can't access this documents. For denied those document to users access we could configure an Access Control List on router (Router1).  After configured ACL all other traffic going to server will be blocked. Now we can ensure that only authorized user can access documents.


There are two types of ACL’s:-
 Standard Access List
Extended Access List
Named Access List
Standard Access List:-
By Standard Access List we can filter only on the source IP address of packet. These type of access list are not powerful as extend access list, but they are less processor intensive for the router. The standard access list identification number between 1 to 99 or 1300 to 1999. Its blocks the whole network and implements near the destination.
Extend Access List:-
In extend access list we can more precise in our filtering. We can evaluate source and destination IP addresses and source and destination port. The extend access list identification number between 100 to 199 or 2000 to 2699. It’s harder to configure, require more processor intensive as standard access list. Extend access list allows much more granular level of control.


Named Access list:-Name access list allow standard and extended access to be given names instead of numbers. It provides some features like name is case sensitive and name can’t start with number or space. We can delete individual statement in a named access list.
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HTTPS:-The HTTPS stands for Hyper Text Transfer Protocol Secure. HTTPS is secure version of HTTP. By the HTTPS, secure communication establishes between clients and servers. HTTPS uses encryption. The HTTPS uses TCL (Transport Layer Security) or SSL (Secure Socket Layer) protocol for make communication encrypted and HTTPS uses TCP port 443 for encryption.


It mostly used to protect sensitive information like username and password or used for payment transaction on the World Wide Web (www) and also protect e-mail and corporate information systems. HTTPS (Hyper Text Transfer Protocol Secure) is commonly used to create secure channel over the internet. 
By default most of the traffic is unencrypted and hackers easily do sniffing attacks for getting information. The HTTPS usually not used on the entire website in internet because the encryptions slow down the website speed.
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HTTP:-
The HTTP stands for Hyper Text Transfer Protocol. HTTP is a client-server protocol that allows requesting web pages from web server for client. HTTP is an applicationlayer protocol widely used on internet. Web servers usually use TCP(Transmission Control Protocol) port 80.
The version of HTTP in common use published in 1997 and defined in RFC 2068 .The HTTP client and web server use request-response method to communicate with each other’s. Client sends the HTTP request and server responding with HTTP responses.How HTTP works:-
Client mostly using different-different web browsers (like Chrome, Firefox and Internet Explorer etc.). When a user wants to access any web page then it types web page address with http (like http://www.knowitinfo.info
and the browser sends an HTTP request message to the server. Now the server response with the requested web page and the client easily accesses the web page.
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SMTP (Simple Mail Transfer Protocol) uses some commands and the responses to transfer message between an MTA Client and MTA Server.

Commands:-1.    Hello:-
This command used by the client to identify itself.
For example - user sends own email with HELO.
HELO: client@email.com (Client mail)
2.    MAIL From:-
This command used by the client to identify the sender of the message.
Example: MAIL From: sender@email.com (Sender mail)
3.    RCPT To:-
This command used for identify the receiver.
Example: RCPT To: identify@email.com
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: identify@email.com
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 (HELO:client@email.com). 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.
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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.
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STP:-
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 SW3receives 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.
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DHCP:-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 169.254.0.1 to 169.254.255.254.
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.
 

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