Airlink X EV-DO - Manual (Page 32)

Raven X EV-DO for Verizon - User Guide, version 2.34 26
Data Communication and Host Modes
DHCP and Routing
DHCP is built on a client-server model. The client broadcasts on the local physical subnet to find
available DHCP servers (generally only one active per network). The server, when a request is
received, reserves an IP address for the requesting client and then sends an IP lease offer to the
client which contains the client's MAC address, followed by the IP address that the server is offer-
ing, the subnet mask, the lease duration, other IP configuration options, and the IP address of the
DHCP server making the offer. Upon receipt of an offer, the client configures its interface
accordingly.
Routing, at its most basic level, is the process of forwarding data on to the correct destination.
One component of routing is address determination, directing data to the correct address either as
its final destination or so it can be forwarded on. Selecting gateways where the data can be
directed is another important component of routing.
The Raven X acts as a one to one gateway forwarding messages to and from one device that is
connected to it. The Raven X does not provide routing for any more than that one device.
DHCP in the Raven X using Public Mode
1. When the Raven X registers on the cellular network, it is assigned an IP address from Verizon,
let’s say A.B.C.D.
2. Acting as a DHCP server, in Public Mode, when the Raven X receives a DHCP request from
an Ethernet device, it hands off the assigned address to the device and sets up the default gate-
way address as A.B.C.1. If the fourth octet is already a 1, it assigns A.B.C.2 as the router
address.
3. The Raven X also sends a /24 netmask (255.255.255.0) and sets up a static route which maps
192.168.13.31 (or the address configured with *HOSTPEERIP if it is changed) to A.B.C.1 (or
A.B.C.2 if that was what the gateway address was given as).
Private Mode allows more direct control of the gateway address, device address, and netmask that
is given out by the modem’s DHCP server. The IP Address that would be assigned to the end
device is configured in the Private Mode settings (see above). Some applications which rely on
specific IP addressing for their operation may have issues working in Private Mode when the
modem has a non-static IP address.
PPPoE with DHCP
When PPPoE is used with the Raven X, DHCP is not needed. A tunnel is set up connecting a
device (such as your computer or a router) with the modem. The device will then simply use the
Raven X’s MAC address to send all outgoing packets. To configure your Raven X and your com-
puter to work with PPPoE, refer to the appendix for PPPoE, page 103.
The AirLink Modem as a Gateway
The primary purpose of the is to forward data from a single device connected to one of the ports to
the network and, ultimately, under most circumstances, to the Internet in a one to one gateway
configuration.