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AT-WA7500 and AT-WA7501 Installation and User’s Guide
75
Unsupported DHCP Server Options
When the access point is acting as a DHCP server, it does not support any
DHCP options other than those listed. The DHCP server disregards any
DHCP options that are not explicitly required by the DHCP specification.
The DHCP server ignores all frames with a non-zero giaddr (gateway IP
address). The DHCP server only responds to requests from its own
subnet.
About Network Address Translation (NAT)
NAT allows IP addresses to be used by more than one end device. The
access point can act as a NAT server, which instantaneously rewrites IP
addresses and port numbers in IP headers so that frames all appear to be
coming from (or going to) the single IP address of the access point instead
of the actual source or destination.
When an end device uses the access point as an IP router, the access
point replaces the IP header, which includes the device MAC address, IP
source address, and TCP/UDP port, with its own. You can configure the
DHCP server to indicate that the access point is the IP router when the
server allocates an IP address. Special consideration is given to changing
the FTP data connection TCP port number, which is in the body of the
TCP frame. After the frame source is modified, it is forwarded to the proper
subnet.
If the destination subnet is a different subnet from the one the access point
is on, the destination MAC address is changed to the IP router that has
been configured for the access point. If the destination subnet is the same
subnet as the one the access point is on, the access point converts the
MAC address to the MAC address that belongs to the destination IP
address. This may involve using ARP for MAC address discovery.
When the access point receives a frame with its IP address, it identifies the
need for address translation by inspecting the destination port number. If
the port number is within the pool reserved for NAT operation, it looks up
the original MAC address, IP address, and port number. The frame is then
modified and forwarded to the end device.
NAT operation is disabled or enabled automatically depending on the
continuous range of addresses you enter into the DHCP server. NAT is
disabled if the range of addresses to be given to DHCP clients is on the
same subnet as the access point. NAT is enabled if the range of
addresses to be given to DHCP clients is not on the same subnet as the
access point; thus, you are creating a virtual network and the DHCP server
will also perform NAT translation.
When NAT operation is enabled, the access point uses the low address in
the range of addresses as its own. The DHCP/NAT clients also use this
address as their router IP address. These clients can configure the access
point using this internal IP address or the normal external IP address.