Port 7615 is the preferred choice and you can think of it as the standard ISL Online port, just like 22 is ssh, 23 telnet, 25 smtp, 3389 rdp etc. - you can also find it in the list of ports at wikipedia.
Having a specific port is also very convenient - if you have a certain company policy and use a firewall/proxy, but would like to adjust it to allow ISL Online traffic, we simply instruct you to open port 7615 and all ISL Online products will work great. If we only used 80/443, the admin would find it difficult to try and allow ISL Online but limit other traffic that also goes through port 80/443.
In general, the first thing to consider when dealing with a proxy environment is to check with the system/network administrator if it is possible to make an exception. This does not mean that you completely disable the proxy, just let the ISL Online traffic through directly and keep filtering the rest. If the proxy supports DNS name exceptions, then allow direct outgoing tcp connection for port 7615 to *.islonline.net. If the proxy supports only IP number exceptions, check this link for a current generated list of our server IPs (note these IPs can change from time to time so you may need to recheck the link if you require them in the future) . Direct connection offers best performance and minimum delays.
In an ideal world of direct connections and flexible security policies, the story would end here, but since there are many customers behind corporate firewalls/proxies where only http and https traffic is allowed (so, port 80 and/or 443) and system/network administrators do not want or are not allowed to add exceptions, we also support that and our applications try to find a working transport even in those situations (detect proxy settings, use wininet, create a tunnel, make use of the wildcard dns - helps with some proxies, etc.).
Situations where such filtering is involved can suffer from additional delays due to transport timeouts in the connection establishment process. ISL Online products always (well, unless you force a certain transport type through registry or command line) try direct connection using port 7615 and if that fails, they try ports 80 and 443 with various proxy methods. Each transport type has a timeout of 20 seconds and on windows we try 8 transport types, so if it is the last one that gets through, this means almost 3 minute delay. If a customer complains about long delays, the best thing to do is connect to the problematic computer and click find best transport in our utility ISL Tester. It will show you a list of successful transports along with the average transfer rate, delays etc. These results will allow you to force the best transport. Both you and your customer will appreciate the reduced connection delay.
If you need help when dealing with ISL Online products in proxy environments you can contact ISL Online team over the telephone, via e-mail or through our live chat (simply go to www.islonline.com and click the Live Chat button in the bottom-right corner of the page).