The remote host will reply with a WILL or a WONT command. IAC DONT xxx means the sender doesn't want the remote side to do option xxx. The remote host will reply with a DO or a DONT command. IAC DO xxx means the sender wants the remote side to do option xxx. IAC WONT xxx means the sender won't do option xxx.

IAC WILL xxx means the sender will do option xxx. Part of an option negotiation, indicating that the sender will do an option, i.e. IAC GA is the go-ahead signal sent by servers (see SUPPRESS-GO-AHEAD option). The SE character terminates the subnegotiation begun with IAC SB. Subnegotiation is used to send option values after both sides agree to an option (see Terminal-Type and NAWS examples in the Option Negotiation Examples Section). The SB (Subnegotiation) character is used following the IAC to begin a subnegotiation. IAC WILL xxx, IAC WONT xxx, IAC DO xxx, or IAC DONT xxx), a subnegotiation of the form: IAC SB SE, or a go-ahead signal of the form: IAC GA.

What follows is either a two-byte option negotiation sequence, (i.e. The Telnet IAC (Interpret As Command) character works like an escape character. The negotiation relies upon a set of special characters described in the table below: The standard for Telnet option negotiation is specified in RFC 854, with additional information in RFC 855. The Telnet session can end when the user logs out, when the server's host has to be rebooted or shut down, or when the user forcibly closes the Telnet client (via Alt-X or something similar). In this way, it is possible for the user to access a remote workstation as if he were logged into it directly (i.e. When the Telnet server sends characters to the client, the client prints them on the screen. The Telnet client waits for the user to enter characters at the console, and sends the characters over the network to the Telnet server.

Telnet server vs client