switch with modem

I connected my PC and the DSL modem from my ISP into one of the 2950 switch ports for each one of the devices. I got internet access but I don't have a full understanding as to how it worked.

The settings were :

PC(static) : IP Address : 192.168.1.6
Gateway : 192.168.1.1
DNS : 208.67.222.222

Switch : 192.168.1.9

DSL Modem : 192.168.1.1

Now, I assume that when I try to access a website like google.com, my pc would attempt to contact the DNS server 208.67.222.222, see that it is in another subnet, which in turn will contact the default gateway which is the ISP modem, sends a ARP broadcast, modem replies with its MAC address, PC sends the request, modem gets it and does some NAT and using a default static route (0.0.0.0 | 0.0.0.0),contacts the DNS servers --- and this part is where I'm kinda confused.....

1.)Does my ISP modem act as a router with NAT and a default static route preconfigured?

Secondly, when I set my PC to "automatically obtain an IP address", I got it so how exactly does that work? Does the switch as as a dhcp server or something?

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Comments

Akiii

That's a local private ip address wich cannot be routed to the internet so yes you're modem acts as a NATting device!. And if you don't have any external dhcp server then probably your LAN interface on the modem is configured for acting as a dhcp.

chX

What model/make is the modem? A lot of modems these days are actually integrated devices, i.e. a modem + router + LAN switch.

I'd be quite confident that the modem is providing DHCP and NAT. The modem/router would have a default route pointing all traffic towards its WAN interface.

Here's what I found on my Billion 7401VGP R3 (Modem/Router)

admin> ip list routes

IP routes:

ID | Name | Destination | Netmask | Gateway / Interface
1 | ppproute1 | 0.0.0.0 | 0.0.0.0 | ipwan

ipwan is the ADSL WAN interface.

Your understanding of how a frame would be handled seems accurate from what I can tell.

veritas_libertas

chX wrote: »

What model/make is the modem? A lot of modems these days are actually integrated devices, i.e. a modem + router + LAN switch.

I'd be quite confident that the modem is providing DHCP and NAT. The modem/router would have a default route pointing all traffic towards its WAN interface.

Here's what I found on my Billion 7401VGP R3 (Modem/Router)

ipwan is the ADSL WAN interface.

Your understanding of how a frame would be handled seems accurate from what I can tell.

Yes, and sometimes: modem+router+switch+wireless+firewall

veritas_libertas

impz wrote: »

I connected my PC and the DSL modem from my ISP into one of the 2950 switch ports for each one of the devices. I got internet access but I don't have a full understanding as to how it worked.

The settings were :

PC(static) : IP Address : 192.168.1.6
Gateway : 192.168.1.1
DNS : 208.67.222.222

Switch : 192.168.1.9

DSL Modem : 192.168.1.1

Now, I assume that when I try to access a website like google.com, my pc would attempt to contact the DNS server 208.67.222.222, see that it is in another subnet, which in turn will contact the default gateway which is the ISP modem, sends a ARP broadcast, modem replies with its MAC address, PC sends the request, modem gets it and does some NAT and using a default static route (0.0.0.0 | 0.0.0.0),contacts the DNS servers --- and this part is where I'm kinda confused.....

1.)Does my ISP modem act as a router with NAT and a default static route preconfigured?

Secondly, when I set my PC to "automatically obtain an IP address", I got it so how exactly does that work? Does the switch as as a dhcp server or something?

Did you turn DHCP on? Unless you do I don't believe you would be assigned those DNS servers.

impz

After playing around with the dsl modem for a bit, I managed to find a table showing IPs leased to various other devices connected to the switch so I would assume that the modem was the DHCP server.

Also, I think I found the default static route, not too sure tho :

Destination |Gateway |Subnet Mask | Interface |
0.0.0.0 |18x.xxx.xxx.xxx |0.0.0.0 |pppoa_0_35_1 |