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Cisco Packet Tracer gives "Invalid IP Address" when I assign an address to a router

        I was given an exercise where I have an IP address range and I have to configure multiple sub networks from that range. The address is <code>238.61.192.128</code> with the mask <code>255.255.255.128 (/25)</code>. I created my first subnet (on paper). It has 32 addresses and it goes from <code>238.61.192.128</code> to <code>238.61.192.159</code> (so the mask is <code>255.255.255.224 (/27)</code>). Then I tried to assign the first usable address (<code>238.61.192.129</code>) to a router (in Cisco Packet Tracer), but no matter what I do, I get an 'Invalid IP Address' error from Packet Tracer. I tried everything, I even tried changing the ports from FastEthernet to Gigabit and the other way around, I thought that might have something to do with it, but nothing seemed to work. Even if I try to create a subnet which just links 2 routers (so no PCs involved) it still doesn't allow me to use that address, I get the same error. I read <a href="https://networkengineering.stackexchange.com/questions/74005/cisco-packet-tracer-invalid-ip-address">here</a> that the range I was given is actually a class D range, so I cannot use it for my purposes (if I understood that question and answer correctly). But how come that exercise gave me this IP range? Is the exercise incorrect? Or can I do something to use that IP range?

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