ZyXEL Communications NXC5200 User Manual page 414

Chapter 28 Services
Both TCP and UDP use ports to identify the source and destination. Each port is a
16-bit number. Some port numbers have been standardized and are used by low-
level system processes; many others have no particular meaning.
Unlike TCP and UDP, Internet Control Message Protocol (ICMP, IP protocol 1) is
mainly used to send error messages or to investigate problems. For example,
ICMP is used to send the response if a computer cannot be reached. Another use
is ping. ICMP does not guarantee delivery, but networks often treat ICMP
messages differently, sometimes looking at the message itself to decide where to
send it.
Service Objects and Service Groups
Use service objects to define IP protocols.
• TCP applications
• UDP applications
• ICMP messages
• user-defined services (for other types of IP protocols)
These objects are used in policy routes, firewall rules, and IDP profiles.
Use service groups when you want to create the same rule for several services,
instead of creating separate rules for each service. Service groups may consist of
services and other service groups. The sequence of members in the service group
is not important.
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NXC5200 User's Guide