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What is NAT

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Network Address Translation (NAT) modifies IP address information in packet headers as they pass through a router or firewall.

Types of NAT

SNAT (Source NAT)

  • Changes the source IP address of outgoing packets
  • Used when internal hosts need to access the internet using a single public IP
  • Also called IP masquerading or PAT (Port Address Translation)

DNAT (Destination NAT)

  • Changes the destination IP address of incoming packets
  • Used to forward traffic from a public IP to an internal server
  • On FortiGate, this is done with VIPs (Virtual IPs)

NAT in FortiGate session output:

  • act=snat — source NAT applied
  • act=dnat — destination NAT applied
  • hook=pre — applied before forwarding (ingress)
  • hook=post — applied after forwarding (egress)

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