connect 3550 Cable modem (dynamic ip)

[Layer_Two]

Hi,

I have a Home Virgin media cable connection at home.

As far as I could understand so far, the only way to get this working with my Cisco gear is by putting the superhub on modem mode.

On modem mode, it becomes just a dumb modem assigning and IP via DHCP

I can get the switch to get an IP address from the modem with

no switchport

ip address dhcp

then I have created

0.0.0.0 0.0.0.0 fa0/48

to route traffic to the port connected to the mode,

The switch gets the assigned Ip address from the modem but it does not route internet traffic.

From the different vlans and subnets on the switch, I can ping the modem.

From the switch I can ping 8.8.8.8

From the machines, i cannot ping 8.8.8.8

On my machines, what should be the gateway? the vlan SVI, the modem's IP address, The fa0/48 ip address? I have tried them all with no success.

Thanks

[root_taker · Accepted solution]

Typically you only get a single internet routable IP address from Cable providers particularly with home services that I am familiar with...  That IP was assigned to the switch via DHCP.  Those switches don't do NAT that I know of.  You will need a device to take the internet routable IP and NAT/PAT it to all your internal hosts. 

Aside from that I would some more details.

[ralphcarter · Accepted solution]

Adam is correct, you need a router in between the switch and modem to perform the NAT/PAT. Your able to ping from the switch because the switch is using the Public DHCP IP that was provided as the source ip for the ping. Your internal hosts have private addresses which are not translated (PAT) to the Public DHCP IP that the switch received.