Actually I now have an example of what I was on about.
I've spent the week at my Dad's place, using my iPad with his WiFi. There was clearly some kind of DNS imcompatiility between the AOL DNS servers and the iPad as about one in three web pages would fail to load 'server not found'. My sister's iPad behaved same as mine, whereas Dad's PC worked fine.
I resolved it by going into the iPad's WiFi settings, choosing Dad's network. The DNS showing 192.168.1.254 (Dad's router) so I changed it in iPad settings to 8.8.8.8, 8.8.4.4, google's DNS servers. The change was 'sticky', and the iPad continued to use Google DNS, and to show that under settings. The iPad now worked... but would the change also affect it when used on orher networks...?
Well I'm now home again and, on my home network, the iPad is showing 192.168.1.1, ie it's letting my router do the DNS, as it did before. The settings for Dad's WiFi have not affected those for my own WiFi.
I would therefor assume (but have not proven) that other settings, such as a fixed IP could also be set for one WiFi network, but left to default to DHCP when on other WiFis?