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Here is the situation. I recentely got a second wireless router that I connected to my original wireless router via an ethernet cable in order to extend the reach of my wireless network across my house. I learned that I had to disable DHCP and give the router an IP address that would not be assigned to any other device. Because my original router had 192.168.1.254, I chose 192.168.1.255 for the second one. Since I am not much of a network wiz, I was happy to see that it actually worked like a charm. Afterwards, I learned that 192.168.1.255 is actually a bad choice because it is the default broadcast address on a network. The problem is that nowhere can I find a detailed explanation of why it is a bad choice, i.e. what are the practical implications of using it for the other access point? Could you give me specific use cases that exemplify the problems? Thx :)

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    In case you want to type your own answer: en.wikipedia.org/wiki/IPv4#Addresses_ending_in_0_or_255 ;) Jan 5, 2015 at 22:18
  • Thanks, but it doesn't really answer my question sufficiently (and in an easy to understand manner). Also, the pararaph almost made it sound like it is not a problem at all. What I want to know is: which problematic situations could arise in a real world scenario? Jan 5, 2015 at 22:22
  • Hmm, a router that doesn't check the Net Mask when you set the device IP address and limit the assigned address to valid numbers. Might want to tell us the brand and model number so we can avoid it like the plague. Unless you're using some oddball CIDR Mask on the internal addressing. Jan 5, 2015 at 22:22
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    because it is the default broadcast address on a network - Completely depends on your subnet mask. If you happened to have a /22 mask it would be a perfectly valid address.
    – Zoredache
    Jan 5, 2015 at 22:41
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    So, what is your subnet mask then?
    – Daniel B
    Jan 5, 2015 at 23:39

2 Answers 2

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I learned that I had to disable DHCP and give the router an IP address that would not be assigned to any other device.

The first thing you have realize here is that you have basically configured your 'wireless router' (aka AP) as a wireless bridge. Bridging the Ethernet to Wifi. Your AP does not actually require an valid IP address on your network at all for this functionality. This bridging is purely layer 2, no IP address is required. In this setup the assigned IP is on used to managed the configuration of the AP (SSID,Encryption, etc). In your setup assigning this address is most likely not much of a problem, since after the initial configuration nothing needs to communicate with this device using TCP/IP.

I find a detailed explanation of why it is a bad choice

Well, the specs say you should not do this, so you shouldn't. Many systems will not even let you do it. If you are able to configure a system this way you will get unusual results.

  • Ethernet devices sending to a subnet broadcast address will set the layer 2 destination to be an Ethernet broadcast. Meaning that packets to that address are transmitted to every single system on the network. A switch/bridge will not filter out broadcasts at all.
  • You may have unusual arp behavior.
  • The system configured with the broadcast address may reply to actual 'broadcasts' that weren't actually meant for it with unexpected replies. For example it might reply with connection refused messages or something else for a broadcast message in a way that the system sending the original broadcast didn't expect.
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I'm with Zoredache as giving the best overall answer to the question as asked.

But nobody has mentioned the fact that Computer Browser in Win 8/10 also uses this address for Discovery.

You can verify this by using Advanced IP Scanner. You'll note that one of your PCs are listed with their primary IP address and also at 192.168.x.255

When you get the IP Scan results you can then use the reported MAC addresses to figure out which of your computers is doing it. You will see that your PC is listed by name and group. Then you will see the Group includes MSBROWSE.

nmap reports the system as unresponsive at that address.

For more information about this behavior, check this out: http://windowsitpro.com/networking/knowing-angles-netbios-suffixes

Cheers

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    – bwDraco
    Mar 11, 2016 at 3:16
  • Understood. Thank you for treating it gently like this. I was careful to link to a site where I knew it wouldn't come with malware of any kind. But I certainly will understand if you changed links to simply the program's name or something. And I will do the same from now on. Your readers should be at a certain level of expertise to be able to find the programs on their own. I'm not selling anything; I'm retired after decades of experience. Your site is quite enjoyable. Thank you for helping to keep it clean. Cheers.
    – SDsolar
    Mar 12, 2016 at 17:24

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