Changing Subnet mask in Airport Utility

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Level 3

825 points

Changing Subnet mask in Airport Utility

Hi,

Background:

I've just taken receipt of a Linksys NSLU2 "Slug". It's essentially a small diskless NAS that allows you to plug in and share USB hard drives over the network. It passes its own POST, and is connected via a fully-functional Cat6 ethernet cable to an Airport Extreme Base Station 802.11n (hereafter simply, "AEBS") running firmware 7.6.1.

In a nutshell: I can't make the Linksys NSLU2 (hereafter known as simply, "the slug") available over the network.

The problem appears to be this: the slug requires that the LAN use a subnet mask of 255.255.255.0. This is its default setting and, if it can be changed, I have to get it connected to the network first. The AEBS currently reports a subnet mask of 255.255.252.0. So far as I have been informed, it is this subnet mask mismatch between the slug and the AEBS that is preventing connectivity.

I'm currently using Airport Utility 5, simply because I'm more accustomed to its settings. If I launch it, authenticate into Manual Setup, and navigate to Internet>TCP/IP, it displays the following information:

• Configure IPv4: USING DHCP

• IP Address: <A REDACTED PUBLIC IP ADDRESS>

• Subnet Mask: 255.255.252.0

• Router Address: <A REDACTED PUBLIC IP ADDRESS VERY SIMILAR TO THE IP ADDRESS ABOVE>

etc.

Under Configure IPv4, I can choose MANUALLY, allowing me to edit the Subnet Mask.

Actual problem:

If I edit the Subnet Mask to 255.255.255.0, and click UPDATE, I get the following error message:

The router address you have entered is not compatible with your WAN IP address

It then lets me change the Router Address before continuing. However, neither I nor my friend (whom I do believe knows more than a fair bit about networking) have any idea what this Router Address is. It is in no way similar to the router's Private IP Address assigned using DHCP ranges. It is a Public IP Address, and shares the first two octets with my IP Address. Because neither I nor my friend know what this represents, or what it is for, we're reluctant to mess with it. But until we "mess" with it, we can't change the subnet mask on the AEBS, and thus I can't access the slug!

Can anyone share some useful information on this matter?

Mac mini (Mid 2011), Mac OS X (10.7.4)

Posted on Jun 22, 2012 5:34 PM

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Answer 1

Jun 24, 2012 3:13 PM in response to Scotch_Brawth

I don't have an AEBS so can only give generic network advice.

I have just had a look at a User Manual for a LinkSys NLSU2 and it states its default setting is an IP address of 192.168.1.77, it does not mention subnet mask I would think it is /24.

You could connect your Mac to the NAS directly with a Ethernet cable, you will probably require a crossover cable. Set you Mac to manual IP in the range of 192.168.1.x/24 and use a web browser to connect to the NAS. You may then be able to change the subnet mask to what you require.

If you are using the Mac as a server it may be better to use another Mac or PC to do this, changing the IP address of a server can cause problems.

Hope I understand your post correctly but I am not sure why you require a /22 subnet mask on an internal network, that subnet mask would allow unto 1023 devices on the LAN.

regards

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Answer 2

Aug 30, 2012 10:12 PM in response to Scotch_Brawth

This is an old post 2 months or more, but if you're still there and never got an answer:

You cannot change the local network subnet mask for an AEBS. It is defaulted by Apple and the subnet mask is fixed at 255.255.255.0 for a total of 256 available addresses if you are using 192.168.0.x, as far as I understand.

Why they make it unchangeable I don't know, sometimes Apple treats us like children.

You are seeing (and changing) only the WAN or public IP assigned by your ISP. Whatever the IP is they gave you, it needs to resolve to that x.x.252.0 subnet they are using. Thus, you get the error "The router address you have entered is not compatible with your WAN IP address".

Your "slug" is already seeing the AESB's x.x.255.0 mask so if it doesn't connect, there is someting else going on, like a manually fixed local ip class that does not match the AEBS.

If you want to change the range (class A, B or C) of local IP's on the AEBS, you can set any DHCP range from the following and the AEBS will follow suit, taking for itself, the first address for that subnet (again fixed by Apple). I'm not sure what the AEBS sets as the subnet mask for each class, though.

10.x.x.x is Class A

72.16.x.x is Class B

192.168.x.x is Class C

It gets way over my head after this, so I hope I gave you good info as best as I understand it. Bottom line, you cannot change the AEBS local subnet mask the way you are attempting.

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Answer 3

Scotch_Brawth Author

Level 3

825 points

Apr 16, 2013 8:47 AM in response to bummper01

Your "slug" is already seeing the AESB's x.x.255.0 mask so if it doesn't connect, there is someting else going on, like a manually fixed local ip class that does not match the AEBS.

You were bang on. Though it wasn't your post that helped, you reached the same conclusion. I eventually had to get in touch with the original seller of the item to see which IP address he had assigned to the device, and from there on everything worked fine.

Thanks for the response, especially after so much time had expired. 😀

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