MicroNet Technology SP5001A/S - Manual (Page 43)

5.1. Network Interface
LAN IP Address
When the gateway connects to the DSL modem directly and shares
the connection for the LAN devices, this IP Address is assigned for
the LAN devices’ default gateway
If the gateway acts stand alone device and behinds the NAT router,
the LAN IP Address can be leave as it is and disable the NAT
function
WAN IP Address
When your gateway has static IP address or set behind the NAT
router, configure the WAN IP Address, Subnet Mask and Default
routing gateway
together
Subnet Mask
A subnet is an identifiably separate part of an organization's
network. Typically, a subnet may represent all the machines at one
geographic location, in one building, or on the same local area
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