NAT is included as part of a router and is often part of a corporate firewall. Network
administrators create a NAT table that does the global-to-local and local-to-global IP
address mapping. NAT can also be used in conjunction with policy routing. NAT can be
statically defined or it can be set up to dynamically translate from and to a pool of IP
addresses. NAT lets an administrator create tables that map:
! A local IP address to one global IP address statically
! A local IP address to any of a rotating pool of global IP addresses that a company
may have
! A local IP address plus a particular TCP port to a global IP address or one in a pool
of them
! A global IP address to any of a pool of local IP addresses on a round-robin basis
NAT is described in general terms in RFC 1631, which discusses NAT's relationship to
Classless Inter-domain Routing (CIDR) as a way to reduce the IP address depletion
problem. NAT reduces the need for a large amount of publicly known IP addresses by
creating a separation between publicly known and privately known IP addresses. CIDR
aggregates publicly known IP addresses into blocks so that fewer IP addresses are
wasted.
4.5 SNMP (Simple Network Management Protocol)
Simple Network Management Protocol (SNMP) is the protocol governing network
management and the monitoring of network devices and their functions. It is not
necessarily limited to TCP/IP networks.
4.6 SNTP (Simple Network Time Protocol)
Simple Network Time Protocol (SNTP) is a protocol that is used to synchronize computer
clock times in a network of computers. In common with similar protocols, SNTP uses
Coordinated Universal Time (UTC) to synchronize computer clock times to a millisecond,
and sometimes to a fraction of a millisecond.
Accurate time across a network is important for many reasons; even small fractions of a
second can cause problems. For example, distributed procedures depend on
coordinated times to ensure that proper sequences are followed. Security mechanisms
depend on coordinated times across the network. File system updates carried out by a
number of computers also depend on synchronized clock times. Air traffic control
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