Understanding the network.pdf - Back to Home

NAT is commonly used in a fashion similar to the PPP/PAT scenario by small regional
ISPs to provide addressing to dedicated attachment customers. The ISP designates
a small block of routable addresses (instead of a single IP address), which are
shared by the customers'end-stations to establish Internet connections. This is an
attractive approach for ISPs that typically provide mail, Web hosting, DNS, and so
on for small businesses that do not have their own Information Technology (IT)
staffs. Instead of having the ISP dedicate a large address range block from their
(limited) registered IP address space, a small address range (typically a /28 or /29
address range) is provided for host addressing through NAT. The customer then
uses an unregistered Class C or Class B address range to provide local host
addressing. These solutions are particularly well suited to NAT's functional design,
which is geared toward scenarios where a limited amount of hosts are
communicating outside the local network at any given time.
NOTE
PAT uses a single public IP address to represent all the nonpublic hosts. Each
nonpublic transaction originates on a different UDP/TCP port number. Therefore, it
appears that the single address is the originator of all the requests.
NAT configuration and operation are quite simple. The router is configured with an
outside and inside interface. The outside interface is connected to the external
Internet or intranetwork using a publicly routable IP address. The inside interface is
connected to the local network that is using an "illegal" or unregistered address
range. NAT translations replace the source address of the packet sent by an inside
host with an address taken from a pool of externally valid addresses. The router
keeps track of these translations as they occur. When a response is sent back, the
router replaces the incoming packet's destination address with the inside address of
the host that originated the request.
The fact that the router rewrites the Layer 3 addressing information contained in the
IP datagrams makes a NAT router distinct from a traditional router. Most routers
only perform Layer 2 readdressing, which is a necessary part of the packet
forwarding process.
Configuring NAT
Enabling NAT on a router is a three-step process:
1. Creation of the address translation pool or static entries
2. Creation of the ACL that specifies which inside addresses to translate
3. Enabling NAT on the inside and outside interfaces