New router, but what subnet with Cisco RVS4000

A small company I administer has always had a 10.0.0.100-10.0.0.200 range with 255.0.0.0 subnet. All ip's are fixed and the management says it need to stay so.
They recently changed ISP and got a ADSL modem with has a fixed LAN address of 192.168.2.1 / 255.255.255.0....
So I bought a Cisco RVS4000 router which I thought could solve the problem.

On the router I can setup the WAN port of this router to 192.268.2.1/24. That's ok.
And on the LAN side I can change the IP-range to 10.0.0.100. That's ok too
But the subnets on the LAN side are fixed to a couple of choises from a dropdown box.
I need the 255.0.0.0 subnet, but that's not there ?!
There's only 255.255.255.0 or 255.255.255.128 or 255.255.255.192 or 255.255.255.224 or 255.255.255.240 or 255.255.255.258 and 255.255.255.252 in the dropdown list.

Is there anything I could do to get the 10.0.0.1xx range to work with this router ?



LVL 5
karstiemanAsked:
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OnityConnect With a Mentor Commented:
255.255.255.0 will give you the range of 0-255 (only 1-254 of them actually hosts)

if you want the 1xx range it (100-200) the calculation is 2^n-2 = 100, nearest is 2^7 = 128
so nearest subnet to your needs is 128 as a sign subnet will give you 129-254 usable hosts.

depends on the numbers you need (also i believe there's a typo in you post as 255.255.255.258 is not a valid subnet as far as i know)
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OnityCommented:
by sign i mean single. sorry

cause I'm procrastinating and not doing my own work

Subnet mask               subnet range (usable hosts)                 number of subnets
255.255.255.0             1-254                                                    1
255.255.255.128         1-126                                                    2
255.255.255.192         1-63                                                      4
255.255.255.224         1-31                                                      8
255.255.255.240         1-15                                                      16
255.255.255.248?       1-7                                                        32
255.255.255.252         1-2                                                        64
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sidetrackedCommented:
u will run in to more trouble i'm afraid.

for this setup to work u need to add a route in the ADSL-modem where u point the network of 10.0.0.0 to the wan ip of your Cisco router. is that really possible on the modem?

you could have a different mask on the Cisco router, like 255.255.255.0 if the gateway is within the 10.0.0.1-254 range.
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