What were the specific changes in the communications or network settings between Raspbian 8 versus Raspbian 9?
If you are having a specific networking issue it's better if you add a bit of context to help us with some context.
Off the top of my head only:
Added the predictable interface names option (configurable in raspi-config, disabled by default in some cases, e.g. dist_upgrade)
Quite a lot of new networking chip drivers
If you updated to 9 and your network/wifi stopped working you should verify if (1) is disabled with
ifconfig (your interfaces will have weird new names).
If you are using an old raspbian8 SD in a newer Raspberry, you have updated to 9 but the network doesn't work, your problem is (2), you'll have to grab the missing interface firmware descriptors manually form raspbian's github.