Telnet is an application layer protocol. Users that run a Telnet client program are able to connect to a remote Telnet system. TCP’s destination port number is 23 and is widely used to control routers and switches. The disadvantage to Telnet is the protocol isn’t completely fail-proof because data comes in as plain text making passwords vulnerable to decoding. SSH is a more secure option for remote logins.

File Transfer Protocol (FTP) is used in TCP/IP networks. One of the more popular protocols, FTP is TCP-based. When an FTP client links up with an FTP server, a TCP connection is made with FTP server’s port 21. Data is transmitted over a separate FTP data connection, another TCP connection, established to well-known port 20. This prevents file transfer interference on the control connection.

Trivial File Transfer Protocol (TFTP) is a more simplified file transfer protocol that utilizes a small group of features, doesn’t require a lot of memory to load, and minimal time to program. TFTP uses User Datagram Protocol (UDP), with no verification of connection / delivery and no error recovery (on the transport layer). TFTP does use application layer recovery by embedding a small header between the UDP header and the data.

Simple Network Management Protocol (SNMP) is an application layer protocol that manages IP devices. With SNMP, network administrators can control parameters on a device remotely, and supervise network performance over a given duration. The three versions of SNMP are SNMP version 1 (SNMPv1), SNMP version 2 (SNMPv2), and SNMP version 3 (SNMPv3).

Simple Mail Transfer Protocol (SMTP) provides e-mail services to IP devices over the Internet. Two mail servers will use SMTP to exchange email. After the email is transmitted, users can access their mail from the server and read it. This is done via any mail client, which uses different protocols, such as Post Office Protocol 3 (POP3), to connect to the server. SMTP uses well-known ports TCP port 25 and UDP port 25, while SMTP applications use only TCP port 25.

BOOTP is a protocol that allows a booting host to configure itself with advanced methods in obtaining its IP address, IP gateway, and other data from a remote server. BOOTP allows use of numerous network hosts to be centrally managed on a single server without having to configure each host independently.

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