What layer in the internet protocol suite does the pop3 (post office protocol 3) operate in?

What is TCP/IP?

TCP/IP is a suite of communication protocols that are used to connect devices and transfer data over the Internet.

TCP/IP can also be used as a communications protocol in a private computer network (an intranet or extra net).

TCP/IP uses:

TCP:

• Defines how applications create communication channels.
• Manages when a message is broken down to be transmitted.

IP addresses:

• An IP address identifies the location of a computer on a network.
• Defines how to address and route packets for delivery.

Ports:

• A port is a location on the recipient computer, where data is received.
• While an IP address tells you where to find a particular computer,
• It doesn't tell you specifically where on that computer a particular connection should be made—that's what port numbers are for.

Some port numbers you should know:

• Port 80: The port number most commonly used for HTTP requests.

• For example, when a client makes a request to a web server, this request is usually sent through port 80.

• Port 5432: The port number used by most database systems; default port for PostgreSQL.

There are a number of other common ports you might want to get familiar with:

Port Number & Usage:

20: File Transfer Protocol (FTP) Data Transfer.

21: File Transfer Protocol (FTP) Command Control.

22: Secure Shell (SSH).

23: Telnet - Remote login service, unencrypted text messages.

25: Simple Mail Transfer Protocol (SMTP) Email Routing.

53: Domain Name System (DNS) service.

80: Hypertext Transfer Protocol (HTTP) used in World Wide Web.

110: Post Office Protocol (POP3) used by e-mail clients to retrieve e-mail from a server.

119: Network News Transfer Protocol (NNTP).

123: Network Time Protocol (NTP).

143: Internet Message Access Protocol (IMAP) Management of Digital Mail.

161: Simple Network Management Protocol (SNMP).

194: Internet Relay Chat (IRC).

443: HTTP Secure (HTTPS) HTTP over TLS/SSL.

Common TCP/IP protocols include the following:

• HTTP (Hypertext Transfer Protocol), which handles the communication between a web server and a web browser;

• HTTPS (HTTP Secure), which handles secure communication between a web server and a web browser;

• FTP (File Transfer Protocol), which handles the transmission of files between computers.

How does TCP/IP work?

TCP/IP uses the client-server model of communication in which a user or machine (a client) is provided a service, like sending a webpage, by another computer (a server) in the network.

Collectively, the TCP/IP suite of protocols is classified as stateless, which means each client request is considered new because it is unrelated to previous requests.

Being stateless frees up network paths so they can be used continuously.

The transport layer itself, however, is stateful.

It transmits a single message, and its connection remains in place until all the packets in a message have been received and reassembled at the destination.

The TCP/IP model differs slightly from the seven-layer.

Open Systems Interconnection (OSI) networking model designed after it.

The OSI reference model defines how applications can communicate over a network.

TCP/IP functionality is divided into five layers, each of which includes specific protocols:

1- The application layer provides applications with standardized data exchange.

Its protocols include HTTP, FTP, Post Office Protocol 3 (POP3),

Simple Mail Transfer Protocol (SMTP) and Simple Network Management Protocol (SNMP).

At the application layer, the payload is the actual application data.

2- The transport layer is responsible for maintaining end-to-end communications across the network.

TCP handles communications between hosts and provides flow control, multiplexing, and reliability.

The transport protocols include TCP and User Data-gram Protocol (UDP), which is sometimes used instead of TCP for special purposes.

3- The network layer, also called the internet layer, deals with packets and connects independent networks to transport the packets across network boundaries.

The network layer protocols are IP and Internet Control Message Protocol (ICMP), which is used for error reporting.

4- The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking.

This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer.

The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer.

5- The physical layer, also known as the network interface layer or data link layer, consists of protocols that operate only on a link -- the network component that interconnects nodes or hosts in the network.

The protocols in this lowest layer include Ethernet for local area networks (LAN's) and Address Resolution Protocol (ARP).

Why is TCP/IP important?

TCP/IP is non proprietary and, as a result, is not controlled by any single company.

Therefore, the IP suite can be modified easily.

It is compatible with all operating systems (OSes), so it can communicate with any other system.

The IP suite is also compatible with all types of computer hardware and networks.

TCP/IP is highly scalable and, as a routable protocol, can determine the most efficient path through the network.

It is widely used in current internet architecture.

Uses of TCP/IP:

TCP/IP can be used to provide remote login over the network, for interactive file transfer, to deliver email, to deliver web pages over the network, and to remotely access a server host's file system.

Most broadly, it is used to represent how information changes form as it travels over a network, from the concrete physical layer to the abstract application layer.

It details the basic protocols, or methods of communication, at each layer as information passes through.

Advantages and Disadvantages of TCP/IP:

The advantages of using the TCP/IP model include the following:

• Helps establish a connection between different types of computers.

• Works independently of the OS.

• Supports many routing protocols.

• Uses client-server architecture that is highly scalable.

• Can be operated independently.

• Supports several routing protocols, and
  is lightweight and doesn't place unnecessary strain on a network or computer.

The disadvantages of TCP/IP include the following:

• Complicated to set up and manage.

• The transport layer doesn't guarantee the delivery of packets.
  Isn't easy to replace protocols in TCP/IP.

• Doesn't clearly separate the concepts of services, interfaces, and protocols, so it isn't suitable for describing new technologies in new networks, and
  is especially vulnerable to an SYN (synchronization) attack, which is a type of denial-of-service (DoS) attack in which a bad actor uses TCP/IP.

A protocol is a set of rules and guidelines for communicating data. Rules are defined for each step and process during communication between two or more computers. Networks have to follow these rules to successfully transmit data.

IEEE 802.11 - Address Resolution Protocol (ARP) - Domain Name System (DNS) - Dynamic Host Configuration Protocol (DHCP) - File Transfer Protocol (FTP) - Hypertext Transfer Protocol (HTTP) - Hypertext Transfer Protocol Secure (HTTPS) - Internet Message Access Protocol (IMAP) - Internet Protocol version 4 (IP) - Internet Protocol version 6 (IPv6) - Lightweight Directory Access Protocol (LDAP) - Multi-protocol label switching (MPLS) - Network File System (NFS) - Network Time Protocol (NTP) - Post Office Protocol (POP) - Post Office Protocol version 3 (POP3) - Rapid Spanning Tree Protocol (RSTP) - Real-time Transport Protocol (RTP) - Secure Shell (SSH) - Simple Mail Transfer Protocol (SMTP) - Simple Network Management Protocol (SNMP) - Spanning Tree Protocol (STP) - Syslog Protocol - Transmission Control Protocol (TCP) - User Datagram Protocol (UDP)

• DNS is a hierarchical decentralized naming system for computers, services, or other resources connected to the Internet or a private network. UDP port 53, by default. If the data portion of the UDP packet exceeds the maximum size, then TCP port 53 is used.
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• Applications and Tools
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• References
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• DDNS

• ChangeiP Free Dynamic DNS delivers the essentials DNS features including a free to use subdomain, unlimited URL redirections, and complete port redirection capabilities.
• D-Link - dynamic DNS host name for your D-Link router
• My DDNS is a free solution I developed.
• no-ip allows remote access your computer, DVR, webcam, security camera or any internet connected device easily. Dynamic DNS points an easy to remember hostname to your dynamic IP address.
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• DNS over HTTPS (DoH)

• FTP is a standard network protocol used for the transfer of computer files between a client and server on a computer network.
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• Commands

  binary	Sets the file transfer type to binary. Critical to the successful transfer of files such as Word documents, graphics, or software packages; also works fine with text files (files that contain only ascii text and no formatting).
  get $filename	Transfers a file from a remote computer to the computer you are typing on.
  mget $file1 $file2	Transfers multiple files from a remote computer to the computer you are typing on.
  ls	Lists all the files in your current sub-directory.
  cd	Changes the directory. To move back up to a previous level, use \"cd ..\" (note the space).
  put $filename	Transfers a file from the computer you are typing on to the remote computer.
  mput $file1 $file2	Transfers multiple files from the computer you are typing on to the remote computer.
  pwd	Tells you the present working directory.
  <Ctrl>+h	Deletes a character to the left of the cursor (acts as a backspace). Note: Use the Ctrl key; don't type the word "Ctrl".
  help $command	Displays the definition of a command. Type \"help\" to see a full list of valid commands.
  bye	Ends the FTP session.

• References
  • RFC 959 File Transfer Protocol (FTP)
  • Wireshark
  • networksorcery.com
  • wikipedia

• IMAP is an Internet standard protocol used by e-mail clients to retrieve e-mail messages from a mail server over a TCP/IP connection.
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• References

• IP is the principal communications protocol in the Internet protocol suite for relaying packets across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet.
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• Applications and Tools
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• Special Use Subnets

  0.0.0.0/8	self-identification
  10.0.0.0/8	Private
  127.0.0.0/32	Loopback
  169.254/16	DHCP link local address
  172.16.0.0/12	Private
  192.0.0.0/24	Special Purpose
  192.0.2.0/24	TEST-NET-1
  192.88.99.0/24	6to4 relay anycast
  192.88.99.2/32	6to4 relay anycast
  192.88.99.2/32	6to4 relay anycast
  192.168.0.0/16	Private
  198.18.0.0/15	Interconnect Device
  198.51.100.0/24	TEST-NET-2
  203.0.113.0/24	TEST-NET-3
  224.0.0.0/4	Multicast
  240.0.0.0/4	Reserved
  255.255.255.255/32	Limited broadcast
  fc00::/7	Unique Local Unicast
  fe80::/10	Link Local Unicast
  ff00::/8	Multicast

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• IP Assignment
  
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• References

• LDAP is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network.
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• Commands

  ldapbind	Use ldapbind to authenticate to a directory server or find out if the server is running.
  ldapsearch	Use ldapsearch to search for specific entries in a directory. ldapsearch opens a connection to a directory, authenticates the user performing the operation, searches for the specified entry, and prints the result in a format that the user specifies.
  ldapadd	Use ldapadd to add entries to the directory. ldapadd opens a connection to the directory and authenticates the user. Then it opens the LDIF file supplied as an argument and adds, in succession, each entry in the file.
  ldapdelete	Use ldapdelete to remove leaf entries from a directory. ldapdelete opens a connection to a directory server and authenticates the user. Then it deletes specified entries.
  ldapmodify	Use ldapmodify to modify existing entries. ldapmodify opens a connection to the directory and authenticates the user. Then it opens the LDIF file supplied as an argument and modifies the LDAP entries specified by the file.
  ldapmoddn	Use ldapmoddn to change the RDN of an entry or move an entry or subtree to another location in the directory

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• References

• NFS is a distributed file system protocol allows a user on a client computer to access files over a computer network much like local storage is accessed.
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• References

• POP is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a server in an Internet Protocol (IP) network.
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• Commands

  USER	Your user name for this mail server
  PASS	Your password.
  QUIT	End your session.
  STAT	Number and total size of all messages
  LIST	Message# and size of message
  RETR $message	Retrieve selected message
  DELE $message	Delete selected message
  NOOP	No-op. Keeps you connection open.
  RSET	Reset the mailbox. Undelete deleted messages.

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• Applications and Tools
  • Email Header Reader
  • Spam - my tips
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• References

• SSH is a cryptographic network protocol for operating network services securely over an unsecured network.
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• References

• SMTP is an Internet standard for electronic mail (email) transmission. Some ways to deal with Spam.
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• Commands

  ATRN	Authenticated TURN
  AUTH	Authentication
  BDAT	Binary data
  BURL	Remote content
  DATA	The actual email message to be sent This command is terminated with a line that contains only a
  EHLO	Extended HELO
  ETRN	Extended turn
  EXPN	Expand
  HELO	Identify yourself to the SMTP server.
  HELP	Show available commands
  MAIL	Send mail from email account MAIL FROM:
  NOOP	No-op. Keeps you connection open.
  ONEX	One message transaction only
  QUIT	End session
  RCPT	Send email to recipient RCPT TO:
  RSET	Reset
  SAML	Send and mail
  SEND	Send
  SOML	Send or mail
  STARTTLS
  SUBMITTER	SMTP responsible submitter
  TURN	Turn
  VERB	Verbose
  VRFY	Verify

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• Applications and Tools
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• References

• SNMP is used to communicate management information between the network management stations and the agents in the network elements.
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• Overview
• Real-time Traps
• Severity: Some vendors supply their recommendation. However, due to lawyers and an inability to know your design, their recommendation may not be valid.
• Port: UDP 161 and 162
• Security: If using v3, then password protected.
• Function: Management and Monitoring.
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• SNMP Security Levels

Version	Level	Authentication	Encryption	What Happens
v1	noAuthNoPriv	Community String	No	Uses a community string match for authentication.
v2c	noAuthNoPriv	Community String	No	Uses a community string match for authentication.
v3	noAuthNoPriv	Username	No	Uses a username match for authentication.
v3	authNoPriv	Message Digest Algorithm 5 (MD5) or Secure Hash Algorithm (SHA)	No	Provides authentication based on the Hashed Message Authentication Code (HMAC)-MD5 or HMAC-SHA algorithms.
v3	authPriv	Message Digest Algorithm 5 (MD5) or Secure Hash Algorithm (SHA)	Data Encryption Standard (DES)	Provides authentication based on the HMAC-MD5 or HMAC-SHA algorithms. In addition to authentication, provides DES 56-bit encryption based on the Cipher Block Chaining (CBC)-DES (DES-56) standard.

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• Applications and Tools
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• References

• STP is a network protocol that builds a logical loop-free topology for Ethernet networks.
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• References
  • IEEE 802.1D IEEE standard for local and metropolitan area networks--Media access control (MAC) Bridges (for STP and RSTP).
  • IEEE 802.1Q IEEE Standards for local and metropolitan area networks-—Virtual Bridged Local Area Networks (for MSTP).
  • Wireshark
  • wikipedia

• UDP allows computer applications to send messages to other hosts on an Internet Protocol (IP) network. UDP has no handshaking. UDP packets are dropped on congested networks. Therefore, it is unclear if the message is delivered.
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• References
  • RFC 768 User Datagram Protocol
  • Wireshark
  • networksorcery.com
  • wikipedia

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