Default gateways and routing

Consider this scenario:

Computer A and computer B are connected to a cable modem/router. They are both configured to use it as the default gateway. Computer C is connected to computer A's second NIC. It is set up to use computer A as its default gateway. All computers are running Windows XP with ICS enabled.

1. Will computer C be able to use the internet?
2. Will computer C and computer B be able to communicate with this configuration?
3. Are there any requirements for certain subnet masks and IPs here? From what I've been reading, computer A is acting as a router between two subnets, so doesn't that mean the IP address has to show that?

1. yes computer c will get internet access thru ics and computer A

2. no B will not be able to communicate. B is on a subnet in common with the router. C is on its own subnet. It will only be able to communcate with A which shares the subnet c is on.

3. the subnet on the 2nd nic on A for C's connection has to be different then that of A's connection to the router.

So let's say all our computers have a subnet mask of 255.255.255.0, and we statically address the nodes as follows:

ADSL router: 192.168.0.1
Computer B: 192.168.0.2
Computer A1: 192.168.0.3
Computer A2: 192.168.1.1
Computer C: 192.168.1.2

Thus representing the network correctly through IP subnetting. The computers have logical hostnames:

B
A1
A2
C

Questions:
1. B attempts communication with C through the hostname 'C'. What exactly will occur? From what I've read, the hostname resolution is a broadcast on the network segment. But as C is on a different network segment, what will happen?

2. B attempts to communicate with C using C's IP address. Will B even bother using an ARP broadcast, seeing as it knows C is on a different segment (through the IP and subnet mask)?

3. If B (from question 2) didn't try ARP, but instead looked for a router that knew of C, what processes would be involved here? Ie, what broadcasts, what datagrams, etc.

Thank you.

1. B will send a netbios name req broadcast and nothing will respond and you will get an error stating unknown host. to make this work you would need a WINS server (uses unicasts) or a router that has IP-helper enabled.

2. depends how the PC is looking it up but it may arp and the router/gateway will respond with its mac and forward the request.

What if I used a hosts file?


How can I determine what occurs with a particular setup?

Ill try and explain this if I make a mistake then someone feel free to jump in..

The default resolution order for resolving a host name is Local Cached Information -> Hosts File -> DNS Servers -> NetBt (NetBIOS over TCP/IP

Network Cards ( NIC's ) only understand MAC addresses.

Workstations—An IP workstation broadcasts an Address Resolution Protocol (ARP) request every time it needs to locate a new IP address on the network. For example, the command telnet mumble.com translates into an IP address through a Domain Name System (DNS) search, and then an ARP request is broadcast to find the actual station. Generally, IP workstations cache 10 to 100 addresses for about two hours.

If a workstation goes to send a packet to a host outside its network it will not arp broadcast for that host but arp for its default gateway if its MAC is not cached.

Lets say you have a subnetwork and PC1 is on network "A" and PC2 is on subnetwork "B" when PC1 tries to send to PC2 it will look at its own IP address and will not know PC2 is on a subnetwork so it will broadcast ARP. ( this is because its network address is the same all the way up to the subnetwork address it doesnt look past its own )This is why gateways use proxy ARP as the gateway will get the arp broadcast look at its networks and see that PC2 is on its other network interface it will reply back to PC1 with its mac. PC1 will send to gateway which in turn will ARP for PC2's MAC and deliver. PC2 will know that PC1 is not on its network ( because its network address ends before its own) and just send to the gateway which will arp for PC1 if not in its arp table. again in the network a PC will arp to send if its sending outside its network it will send the IP packet to its default gateway's MAC when the response comes back the router will send via layer 2 MAC.

hope this makes sense.

I'm doing the same thing at home, but having some problem.
Computer C not able to connect to internet.
Below are the settings :

Computer A running WINXP PRO (ICS enabled)
Computer C running WINXP HOME
router: 192.168.0.1
Computer A :
NIC1 (to internet)
IP Address. . . . . . . . . . . . : 192.168.0.105
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.0.1
NIC2 (to computer c)
IP Address. . . . . . . . . . . . : 192.168.1.1
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.0.105

Computer C:
IP Address. . . . . . . . . . . . : 192.168.1.2
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.1.1

The setting is it correct ?
Pls help ! Thanks !

Try running the network setup wizard on computer C. It sometimes helps.