We use a lot of these (Lantronix UDS-10 units) at the MMT observatory. They are a little white box (maybe 3x5x1 inches) with a DB-25 serial port on one end and a RJ-45 network connection on the other. There is also a connection for 12V DC power, typically supplied by the ever present "wall-wart". The network is only 10 Mbit, but this is hardly an issue given that the unit is handling traffic throttled by an RS-232 serial connection, running no faster than 38.4 kbit.
As shipped from the factory, the unit has IP number 0.0.0.0. This makes it come up using DHCP to obtain an identity. What I do is keep an entry for new lantronix units in my dhcpd.conf file on a server I have handy access to, and edit the MAC address for the unit into that file for each unit I need to set up.
Once plugged in, it will commence with DHCP traffic on UDP port 68, get its identity, and you will be able to ping it.
Now the thing to do is to telnet to it on port 9999. Hit return and you should be using its configuration menu. I use entry 0 to set the IP number so it will have a fixed identity (you could just continue to use DHCP, but we don't). And I also set a 4 character password (it won't take any more). I do not configure a gateway address. We also place these on our internal 192.168.100. network. With the lack of a gateway and the location on our internal network it will be difficult to access these units from outside of our network.
Once configured, they may be accessed on port 10001 to get a transparent pipe to the RS-232 port. Separate menus can configure baud rate and other serial parameters. This only gets sticky when some serial device is using a Billy-Bob style binary protocol (such as our neslab units).