The firewall protects the servers in your environment by blocking all communications other than what you need. The firewall works to stop undesired communications traffic by shutting down protocol types on certain ports.
Protocol Types
Computers mostly communicate via three protocols of the Internet Protocol suite: TCP, UDP and ICMP. TCP is a used in conjunction with the original Internet Protocol (IP) and hence, is sometimes referred to as TCP/IP. TCP allows for ordered data transmission with two-way communications (i.e. message sent and an acknowledgement sent back). TCP is the most-used protocol for application-level communications. UDP is a one-way broadcast message paradigm that does not guarantee data delivery. ICMP is a low-level protocol used for error messages and a few applications such as Ping and Trace Route.
Port
Computers direct their communications via IP address and a port within that address. Well known ports are 80 for HTTP (i.e. web sites), 443 for HTTPS (i.e. secure web sites) and 25 for STMP (i.e. mail relay).
Firewall Rules
Firewall rules direct the firewall to allow or disallow a certain protocol type (TCP or UDP) on a certain port in a certain direction (inbound or outbound). By default, all protocols and all ports are disallowed certain for port 80 (HTTP) and either port 23 (for SSH tunnels to Linux servers) or port 3389 (for RDP communications to Windows server).
