Introduction to NAT
73
WAN Access Routers Getting Started Guide
7 • Security
Introduction to NAT
The basic steps for configuring NAT are:
1. Enable NAT between the internal and external interfaces of the firewall.
2. Create global addresses which will be added to the global pool of IP addresses on the WAN interface.
3. Create a reserved mapping between a global IP address and the IP address of an internal PC.
A Global Address Pool is a pool of addresses seen from the outside network. Each external interface creates a
Global Address Pool with a single address—the address assigned to that interface. For outbound sessions, an
address is picked from a pool by hashing the source IP address for a pool index and then hashing again for an
address index. For inbound sessions, it is necessary to create a reserved mapping.
A reserved mapping is used so that NAT knows where to route packets on inbound sessions. The reserved map-
ping will map a specific global address and port to an inside address and port. Reserved mappings can also be
used so that different inside hosts can share a global address by mapping different ports to different hosts. For
example, Host A is an FTP server and Host B is a web server. By mapping the FTP port to Host A and the
HTTP port to Host B, both insides hosts can share the same global address. Setting the protocol number to
255 (0xFF) means that the mapping will apply to all protocols. Setting the port number to 65535 (0xFFFF) for
TCP or UDP protocols means that the mapping will apply to all port numbers for that protocol.
Some applications embed address and/or port information in the payload of the packet. The most notorious of
these is FTP. For most applications, it is sufficient to create a trigger with address replacement enabled. How-
ever there are three applications for which a specific Application Level Gateway is provided: FTP, NetBIOS,
and DNS.
Enabling NAT
The configuration of NAT in this example follows on the preceding configuration completed earlier in
this chapter.
1. Go to the Security Interface Configuration page by clicking on Security under Configuration in the menu.
2. Click on Enable NAT to internal interfaces in the Security Interfaces table. NAT is now enabled between
the internal (LAN) and the external (WAN) interfaces of the firewall.
Global address pool and reserved map
1. Click on Advanced NAT Configuration... on the web page, Security Interface Configuration.
2. Click on the hyperlink Add Global Address Pool... The global IP addresses need to be created and put
into the Global Address Pool.
Summary of Contents for LR120A
Page 1: ...LR120A LR121A LR122A LR125A WAN Access Routers Getting Started Guide...
Page 24: ...19 Chapter 2 Product Overview Chapter contents Introduction 20 Applications overview 21...
Page 115: ...110 Appendix B Cable recommendations Chapter contents Ethernet Cable 111 Adapter 111...
Page 128: ......
