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Gateway is on different subnet than IP addresses

sasllc
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Last Modified: 2018-09-07
This small office customer recently changed to AT&T for their ISP.  Looking at ipconfig, their PCs are all 192.168.5.x, but their gateway is 192.168.4.1.  All the PCs get "automatic" IP addresses, and they all access the internet and their local network just fine.

The problem comes in when I need to set new addresses on their two wireless radios.  We always set them up with static IPs.  If I try to set up the static IP as 192.168.5.232, and the gateway as 192.168.4.1, I get an error saying both must be in the same subnet.

Trying to get fast, qualified help from AT&T while I'm here on site is going to be impossible, based on past experience.  I'm hoping someone here at e-e knows what, if anything I can do to get the IP addresses and the gateway in the same subnet, or some other solution that will work.  I am accustomed to seeing both in the subnet everywhere I go, so I'm lost as to why it is this way, and what to do about it.  TIA
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nociSoftware Engineer
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I would look mainly in certain areas: The DHCP settings, along with the actually IP settings of the router.
Dr. KlahnPrincipal Software Engineer
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If I try to set up the static IP as 192.168.5.232, and the gateway as 192.168.4.1, I get an error saying both must be in the same subnet.
Did you ever try setting their IP addresses as 192.168.4.x? Might be a way around things until you get it all sorted. But I would think you could configure the router...
atlas_shudderedSr. Network Engineer
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nociSoftware Engineer
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BTW gateway can never be on a different subnet, that doesn;t work... So if it works the subnet must be bigger than you thought..

@masnrock the netmasks need to be the same even then so not a solution... If the other currently works the conclusion must be that the netmask is smaller (==> the network is bigger).

/32 = 255.255.255.255 = 11111111.11111111.11111111.11111111
...
/23 = 255.255.254.0 = 11111111.11111111.11111110.00000000
/22 = 255.255.252.0 = 11111111.11111111.11111100.00000000
/21 = 255.255.248.0 = 11111111.11111111.11111000.00000000
/20 = 255.255.240.0 = 11111111.11111111.11110000.00000000
/19 = 255.255.224.0 = 11111111.11111111.11100000.00000000
/18 = 255.255.192.0 = 11111111.11111111.11000000.00000000
/17 = 255.255.128.0 = 11111111.11111111.10000000.00000000
....
/0   =  0.0.0.0.0  = 00000000.00000000.00000000.00000000  (aka the internet...)
(the netmask shift one bit to the left each time...)

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By the time I had finished posting this question, I found the business owner on the phone with AT&T--and she insisted that I talk with them.  After FOUR HOURS of holding, being transferred, being disconnected, and trying to communicate in a foreign language, even talking with confused Level 2 techs, I found this morning that nothing had been fixed, because everyone still had 192.168.5.x addresses.  And I could do nothing about it, because this is a "managed router" that AT&T would not let me access.  

Long story short, today I had to set static IPs on several devices in the 192.68.4.x range in order to make them work.  This was something I could not do yesterday, so I guess it is fair to say they AT&T tech support did "something"--but I don't know what.  The customer still has a gateway of 192.168.4.1, and a subnet mask of 255.255.252.0, and most of the PCs still have automatic 192.168.5.x addresses.  But the ones I set static in the 192.168.4.x range now work as well, so at this point I'll have to give up and go with it.
JohnBusiness Consultant (Owner)
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You can always put your own Wireless Router on one subnet and use that subnet for your network. Additional router, but that should work fine.
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Given the way you initially said things were configured, it almost sounds there was a very weird router misconfiguration at the start.

Honestly, the best solution was suggested by Dr Klahn far at the beginning: dump AT&T. If that's the type of support you're getting at the start, it surely isn't going to get any better. What alternatives are available that meet your client's needs?

Your own router is only going to help so much: you would have to opt for a subnet that doesnt overlap with the existing one in the AT&T router. Obviously there is also that double NAT thing.
Mitul PrajapatiIT Supervisor
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atlas_shudderedSr. Network Engineer
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saslic -

Is the carrier running DHCP on the router?
nociSoftware Engineer
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The customer still has a gateway of 192.168.4.1, and a subnet mask of 255.255.252.0
Here you say it... your (sub)NETMASK is 255.255.252.0  so  set THAT on every device.  (/22)

You may now have devices with 192.168.4.x netmask 255.255.255.0 (/24)   ==> those will FAIL for some devices.
on the existing network. (because broadcast addresses dont match for
192.168.4.x/22  = 192.168.7.255   with 192.168.4.x/24 = 192.168.4.255   so anything broadcast based WILL fail in that setup to reach 192.168.5.0 - 192.168.5.254 .  and station @192.168.4.255 might really be interesting... to have as user.

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