Chapter 20 NAT

•Use the Advanced screen to change your NBG4615’s trigger port settings (Section 20.5.3 on page 177).

20.1.2What You Need To Know

The following terms and concepts may help as you read through this chapter.

Inside/Outside

This denotes where a host is located relative to the NBG4615, for example, the computers of your subscribers are the inside hosts, while the web servers on the Internet are the outside hosts.

Global/Local

This denotes the IP address of a host in a packet as the packet traverses a router, for example, the local address refers to the IP address of a host when the packet is in the local network, while the global address refers to the IP address of the host when the same packet is traveling in the WAN side.

Note: Inside/outside refers to the location of a host, while global/local refers to the IP address of a host used in a packet.

An inside local address (ILA) is the IP address of an inside host in a packet when the packet is still in the local network, while an inside global address (IGA) is the IP address of the same inside host when the packet is on the WAN side. The following table summarizes this information.

Table 68

NAT Definitions

 

 

 

ITEM

 

DESCRIPTION

 

 

 

Inside

 

This refers to the host on the LAN.

 

 

 

Outside

 

This refers to the host on the WAN.

 

 

 

Local

 

This refers to the packet address (source or

 

 

destination) as the packet travels on the LAN.

 

 

 

Global

 

This refers to the packet address (source or

 

 

destination) as the packet travels on the WAN.

 

 

 

Note: NAT never changes the IP address (either local or global) of an outside host.

What NAT Does

In the simplest form, NAT changes the source IP address in a packet received from a subscriber (the inside local address) to another (the inside global address) before forwarding the packet to the WAN side. When the response comes back, NAT translates the destination address (the inside global address) back to the inside local address before forwarding it to the original inside host. Note that the IP address (either local or global) of an outside host is never changed.

The global IP addresses for the inside hosts can be either static or dynamically assigned by the ISP. In addition, you can designate servers, for example, a web server and a telnet server, on your local network and make them accessible to the outside world. If you do not define any servers , NAT offers the additional benefit of firewall protection. With no servers defined, your NBG4615 filters out

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NBG4615 User’s Guide