[wellylug] saturn cable connection with linux

Mark mark at incet.com
Mon Feb 4 23:29:26 NZDT 2002


sorry to mislead the rfc I quoted should be rfc 1918 :)) not 1912 which 
is more related to
well know music !!



Mark wrote:

>Hi Don,
>
>This is a little bit of info I am aware of about Hybrid Fibre Coax Networks:
>
>Downspeed bandwidth  is greater than upspeed  about 30MB:10MB I believe
>
>your bandwidth for your modem is rateshaped according to your contract
>
>Network is partitioned and there is an effective broadcast network 
>within the segment
>
>the total bandwidth on the segment is shared between the users on that 
>segment
>
>there is filtering so that if you sniff traffic on the modem you see 
>only your traffic and broadcast traffic (like a switch)
>
>the network acts a layer 2 bridge with filtering, the modem has no ip 
>address for the customer traffic
>you cannot use a hub directly into the back of the modem since you only 
>get 1 real ip
>
>If you try to use a hub and use private IPs on the hub then those 
>machines will not be able to use the internet because the source ip of 
>the packets are in the RFC1912 address space and will be dropped by the 
>internet
>
>if you wish to use > 1 PC on the network configure 1 with Linux and then 
>have 2 network cards and plug the hub into the second card as already 
>described previously and then your setup will work for multiple PCS.
>
>you canot bypass the modem - the cable is broadband (data modulated on a 
>number of carrier frequencies) and ethernet is baseband where the data 
>signals are written to the media without any modulation.  The cable 
>modem does the modulation/demodulation.  An analogy would be connecting 
>your pc serial port to the phoneline!!
>
>any help
>
>mark
>
>
>
>
>Don Jones wrote:
>
>>Hmmm
>>
>>Im getting a saturn cable modem installed on Friday
>>
>>>You plug your ethernet card into the cable modem 
>>>
>>using the ethernet 
>>
>>>cable they provide.
>>>
>>Is this a X-over do you know?
>>
>>>Saturn give you an IP address, a netmask, a 
>>>
>>gateway address and two dns 
>>
>>>servers. 
>>>
>>Whats the netmask?
>>
>>The description suggests that the saturn cable 
>>"modem" is just a bridge/media converter/layer2 
>>device. Has anyone investigated this? Essentially 
>>the whole cable network is one big LAN. This 
>>suggests that anyone who plugs the "modem" strait 
>>into a hub with a number of hosts on it is 
>>essentialy an extension of the lan. Except with a 
>>bridge in between. So presumably it filters 
>>incomming frames at the "modem" based on its 
>>external MAC address. That will allow filtering of 
>>inbound frames (ie you wont be able to sniff traffic 
>>on the main LAN beyond the bridge). But what about 
>>frames comming from internal hosts in the hubbed 
>>setup described above, how does the bridge decide 
>>which frames to forward? Either it forwards then all 
>>and you have internal addresses out on the main 
>>segment or does it do some kind of arp or deny any 
>>ip addresses in the internal range? - whatever - 
>>either would bump it up to a layer 3 device - anyone 
>>know?? 
>>
>>I wonder what the specs of the segment are - is it 
>>real ethernet (anyone know what type?) - 100Mbps or 
>>Gigabit (its probably 1544Mbps or something)? Could 
>>you hack together a connector to bypass the modem - 
>>like a bnc 10base2 card running at whatever the 
>>segments speed is? or some type of coax to cat5 
>>media convertor running at the right speeds.
>>
>>Forgive my ramblings - but Im curious ;)
>>
>>Don Jones
>>
>
>
>
>
>  .-.   Wellington
>  /V\   Linux
> // \\  Users       
>/(   )\ Group
> ^^-^^
>        http://wlug.paradise.net.nz/
>
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