New router, but what subnet with Cisco RVS4000

A small company I administer has always had a range with 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 /
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 That's ok.
And on the LAN side I can change the IP-range to 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 subnet, but that's not there ?!
There's only or or or or or and in the dropdown list.

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

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OnityCommented: 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 is not a valid subnet as far as i know)

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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             1-254                                                    1         1-126                                                    2         1-63                                                      4         1-31                                                      8         1-15                                                      16       1-7                                                        32         1-2                                                        64
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 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 if the gateway is within the range.
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