Switch – How to utilize multiple IP addresses from IP pool given by ISP

nat;routersubnetswitch

I'm still a student so please forgive me for my ignorance. I'm still learning. So I got a T1 line to practice and learn with and they gave me 5 IPs to play with. For example,8.8.8.0/29 8.8.8.[1-5]. Now I have my line going directly into a Sonicwall Pro 1080 set up with an IP of 8.8.8.1. I want to add a Cisco 881 and give it the IP address of 8.8.8.2. I'd like to do it without plugging into the 1080 and use NAT. So I dropped a switch in front of my 1080 to run the lines in parallel, but once I plugged the 881 in, I lost all Internet access through my 1080. Is this something that can be done? Is there a better way of doing this? BTW, the switch I am using is a Buffalo BSL-PS-G2108M.

Best Answer

On the SonicWall 1080 & the Cisco 881 are you using the same Default-gateway (the IP of the T1)? In your example, 8.8.8.0/29... 8.8.8.1 would be the default gateway of the T1 provider. So your sonicwall should be .2, 881 should be .3 (.2-.6 usable IPs) You are correct that your topology/setup 'SHOULD' Work. Although you can run into oversubscription issues on your T1 doing that, but is fine for learning/practice.

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Router – Using IP addresses from IP pool given by ISP

This depends on exactly how the ISP provides them to you. You may have "4 DHCP addresses", a "routed" /30, or a "connected" /29.

For example, Time Warner Cable (business class) allows 4 devices to use DHCP to get an address. (similar to residential service that's limited to one)

"routed" is what you'd see where you have a WAN side and LAN side. This is the traditional "lease line" model where the T1/DS3/etc. ISP side has an independent address to which the static address block is routed; from there they can be assigned as a LAN or NAT'd individually (the firewall situation Bulki mentions.)

"connected" would be where the ISP provides your LAN (i.e. a data center, or the "routed" example where you don't control the router)

Routing – How to aggregate a routing table given IP addresses and their subnet masks

First, separate the entries by next hop. You have to summarize them separately:

IP             SUBNET           NEXT HOP
129.10.112.0   255.255.255.0    R1
129.10.80.0    255.255.255.0    R1

129.10.0.0     255.255.0.0      R2

129.10.64.0    255.255.192.0    R3

129.10.63.0    255.255.255.0    R4
129.10.63.0    255.255.255.0    R4
129.10.65.0    255.255.255.0    R4
129.10.66.0    255.255.255.0    R4

Then for each next next hop, convert all the network addresses to binary. Here is just the first one:

10000001.00001010.01110000.00000000 = 129.10.112.0 
10000001.00001010.01010000.00000000 = 129.10.80.0

Now find all the identical digits, starting from the left. In this case, the digits are all the same up to the 18th position. So your new mask is /18. Now, using either address and the /18 mask, find the network address by ANDing the address and the mask:

10000001.00001010.01110000.00000000 = 129.10.112.0
11111111.11111111.11000000.00000000 = /18 (255.255.192.0)
-------------------------------------
10000001.00001010.01000000.00000000 = 129.10.64.0 /18

So the best summarization of the first two routes is 129.10.64.0/18.

(the rest is left as a exercise for the reader)

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Router – Using IP addresses from IP pool given by ISP

Routing – How to aggregate a routing table given IP addresses and their subnet masks

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