Hy everybody!!

I'm in a lan with IP's like this : 10.10.10.xxx Mask <a class="bbcode_url" href="http://255.255.255.0" target="_blank" rel="nofollow noopener noreferrer">255.255.255.0</a> . I can connect to the network, i can use ping, nmap...
My problem is that when I try to add gw it sais NETWORK IS UNREACHABLE! I have an external IP : 80.96.157.xxx Mask <a class="bbcode_url" href="http://255.255.255.240" target="_blank" rel="nofollow noopener noreferrer">255.255.255.240</a> I can't make it work! I can't connect to the internet. And another thing: how do i calculate the Bcast Address?? :(( any help please? I have RH 8