User guide

Network Address Translation (NAT) Engineering Your IP Communications
158
pmp-0229 (Mar 2013)
When a DHCP Server is Not Found
To operate on a network, a computer requires an IP address, a subnet mask, and possibly a
gateway address. Either a DHCP server automatically assigns this configuration information to a
computer on a network or an operator must input these items.
When a computer is brought on line and a DHCP server is not accessible (such as when the server
is down or the computer is not plugged into the network), Microsoft and Apple operating systems
default to an IP address of 169.254.x.x and a subnet mask of 255.255.0.0 (169.254/16, where /16
indicates that the first 16 bits of the address range are identical among all members of the subnet).
Network Address Translation (NAT)
NAT, DHCP Server, DHCP Client, and DMZ in SM
The system provides NAT (network address translation) for SMs in the following combinations of
NAT and DHCP (Dynamic Host Configuration Protocol):
• NAT Disabled (as in earlier releases)
• NAT with DHCP Client (DHCP selected as the Connection Type of the WAN interface) and
DHCP Server
• NAT with DHCP Client(DHCP selected as the Connection Type of the WAN interface)
• NAT with DHCP Server
• NAT without DHCP
NAT
NAT isolates devices connected to the Ethernet/wired side of an SM from being seen directly from
the wireless side of the SM. With NAT enabled, the SM has an IP address for transport traffic
(separate from its address for management), terminates transport traffic, and allows you to assign a
range of IP addresses to devices that are connected to the Ethernet/wired side of the SM.
In the Cambium system, NAT supports many protocols, including HTTP, ICMP (Internet Control
Message Protocols), and FTP (File Transfer Protocol). For virtual private network (VPN)
implementation, L2TP over IPSec (Level 2 Tunneling Protocol over IP Security) and PPTP (Point
to Point Tunneling Protocol) are supported. See NAT and VPNs on Page 163.
DHCP
DHCP enables a device to be assigned a new IP address and TCP/IP parameters, including a
default gateway, whenever the device reboots. Thus DHCP reduces configuration time, conserves
IP addresses, and allows modules to be moved to a different network within the Cambium system.
In conjunction with the NAT features, each SM provides