How can I update the firmware in my router via tftp? I'm going to flash my wifi-router with alternative firmware by following these instructions on my Mac but I'm struggling with the step

Set Your server's IP-Address to 192.168.1.15/24


I can't understand, what is this server. Is it DNS-server? 
I fI try to change address of DNS-server, than it gets cut to 192.168.1.15.

How do I need to set up my Mac to allow the router to fetch the new firmware via tftp?
192.168.1.15/24 is a so-called slash notation, which specifies both the IP address and the network mask at the same time. It's an alternative but shorter notation, which says: your Mac's IP address should be set to 192.168.1.15 and the network mask to 255.255.255.0. 

In addition to that, you will need to enable the tftp server on your Mac. To do that, look for /System/Library/LaunchDaemons/tftp.plist file. If you find it, you need to edit it, to enable launching the tftp server. Edit the file with your favourite editor (sudo vi or sudo nano should do - this is all best done in the Terminal.app). Inside the file, you'll find the following lines: 

&lt;dict&gt;
    &lt;key&gt;Disabled&lt;/key&gt;
    &lt;true/&gt;


change these to be: 

&lt;dict&gt;
    &lt;key&gt;Disabled&lt;/key&gt;
    &lt;false/&gt;


After you make that change, you'll need to run the following command in Terminal.app, to start the tftp service: 

sudo launchctl load -w /System/Library/LaunchDaemons/tftp.plist

You will then need to put your firmware file into /private/tftpboot, make sure it's world readable (in Terminal.app run chmod 644 /private/tftpboot/filename where filename is the name of the file containing your firmware) and finally point your router to the location of the file, according to your router's instructions.