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Chapter 4: Configuring the High-Level Network

4-122

Address translation: SNATs, NATs, and IP forwarding

The BIG-IP uses address translation and forwarding in various ways to

make nodes accessible that would otherwise be hidden on its internal

VLAN.

◆ A virtual server translates the destination address of an inbound packet

from its own address (the virtual server’s) to the address of the node to

which it load balances the packet. It then translates the origin address of

the reply back to its own address so the originating host will not try to

address the member node directly. This translation is basic to the way the

virtual server works in most configurations and it is enabled by default.

◆ You can configure a SNAT (Secure Network Address Translation) or

NAT (Network Address Translation) to give a node that is a member of a

load balancing pool a routable address as an origin address for purposes

of generating its own outbound traffic. A SNAT can be configured

manually, or automatically using the SNAT auto-map feature.

◆ You can configure a forwarding virtual server to expose selected nodes

to the external network.

◆ You can configure IP forwarding globally to expose all internal nodes to

the external network

For more information on enabling address translation for virtual servers,

refer to Virtual servers, on page 4-69. The following sections describe how

to configure SNATs, NATs, and IP forwarding.

SNATs

A secure network address translation (SNAT) provides a routable alias IP

address that a node can use as its source IP address when making

connections to clients on the external network. Unlike a network

translation address (NAT), a SNAT does not accept inbound traffic, and

this is where its security lies. When you define a SNAT, you can use it in

any of the following ways:

• Assign a single SNAT address to a single node

• Assign a single SNAT address to multiple nodes

• Enable a SNAT for a VLAN

Note that a SNAT address does not necessarily have to be unique; for

example, it can match the IP address of a virtual server.

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