router does not accept /21 subnet mask

[Dimitris_3]

I am using a ZTE H108N ADSL2/ADSL2+ modem and im trying to add a static route to it but the subnet mask wont get accepted.
IP: 37.xxx.xxx.0
subnet mask:255.255.248.0
Is this a valid subnet mask and its getting rejected for some other reason or it is invalid?
Thanks in advance,
Dimitris

 

[bill001g]

Can't say for sure on that router but I have seen a number of these consumer routers that have very restrictive rules on the lan ip and subnet. Some will only accept 192.168.x.x with a /24 they will not even allow you to use other private blocks like 10.x.x.x even with a /24. Also many for whatever reason force you to use private ip blocks. Realistically only huge companies that got ip blocks almost 20yrs ago when it was easy use actual routable ip for their internal network.

Hard to say but you likely can not fix it if that is a restriction. Since it has a adsl modem built in you do not even have the ability to load third party firmware

 

[Dimitris_3]

So im stuck with this restriction i guess? No way to bypass this or something?

 

[bill001g]

I have never used that router and restrictions like this are seldom documented in the manual. I would try 192.168.0.0 /21 and see if it takes that.

I have seen more than 1 router that will only take /24 masks.

Years ago this was suppose to be because the router did not have enough memory to hold the NAT tables for that many ip. Now the memory is huge so that can not be why.

More than likely it is to help out the stupid people who would be confused having to learn what a subnet mask is so they just remove the feature.

 

[Dimitris_3]

I will probably try that and see if it works, but the thing is i was trying a few other routes and i had the same problem with /22 and /20 but after a few tries it accepted both of them.