On Fri, 05 Mar 2004 07:10, Christopher Sawtell wrote: > > The TelstraClear cable modems do not do NAT. > > We may not be talking about the same modems here, but the one I was issued > with by Telstra/Clear/Paradise has a built-in dhcp server. It allocates > upto 32 addresses if it's enabled - T/C/P don't by default. So doesn't that > mean that NAT is available if you want it? I'd appreciate somebody clearing > up my misunderstanding if indeed I have one.
This is a reasonable assumption and I had similar thoughts for a different reason when I first got my cable modem in mid 2001. I have a GE-SB3100 and I note from a later post yours is an SB3200. I believe the various replies on this thread that state the Surfboards do NOT do NAT are correct and this applies to the SB3, 4 and 5nnn series (on the other hand the surfboard SBG1000 wireless cable mode does). The DHCP server in yours and mine is only useful when the cable modem is 'out of service' . In that mode it can allocate up to 32 addresses in the 192.168.100.11 through 42 range, presumably to allow multiple devices on the LAN to communicate with each other while internet connectivity is down. That, however, does not mean it is has (or requires) NAT capability. The surfboards are implemented as a transparent learning bridge (layer 2) refer to IEEE802.1d for details. Devices connected to the ethernet interface are communicate directly with a 'Universal Broadband Router' in T/Cs 'head end' at the MAC layer (layer 2). No address translation or routing is required. If T/C/Paradise were to support more than one node connected to the internal 10Base-T interface via a hub or switch, then each node would need a unique external IP address. Layer 2 packets sent between the CM and UBR would contain a unique MAC address for each node. Mapping from IP address to MAC address is via arp not NAT. One way to to picture this is as though your internal LAN were connected directly to T/C/Ps UBR (that's essentially what a bridge accomplishes - I think). An arp -a should help confirm that. However, as you have noticed the surfboard's protocol stack implements more than would be required for a layer 2 bridge. The stack ALSO implements layer 3 (IP); the address 192.168.100.1 is the IP address of the internal interface while the external CATV interface usually has an IP address in the 172.n.n.n range. The protocol stack supports TCP and UDP and as you have noted has a built in DHCP and tftp server for transferring config files. It also has snmp support for diagnostics, but to my disappointment I discovered this is only active when the CM is reset and disconnected from the HFC network. It looks like T/C disable snmp (via their config file) on both the internal AND external interfaces, which is a pity because there is more info available via snmp than can be viewed via the web admin tool. Hope this helps.
