The host associated with a numeric internet address can be optionally shown in netactview. The internet host names are assigned using the Domain Name System (DNS). The host is related with the website the application is communicating with but not always identical. The host won't tell you the exact web address the application is connecting to (Ex: for http://www.weather.com/weather/today you will see only www.weather.com). For connections made by other computers to the services you have opened on you computer (like shared folders, ftp or ssh), or for peer connections (like bittorrent), the host name will usually be formed by a customer ID or IP and the internet provider name (like customer-100.comcast.net or 10.10.10.10.comcast.net). By default only the remote host is shown. See the View menu in order to toggle the remote and local hosts on or off.
Many websites use host names that relate in non obvious ways with the actual web addresses you are typing in a browser. To get around that you can use the whois service. Netactview does not currently have whois integrated. In order to use whois just copy the remote address from netactview and use options like: Network Tools from Ubuntu (System Tools/Network Tools/Whois - not installed by default), whois ip address command or online whois or ip location web sites.
The host name information is loaded separately from the connection information as it can take a few seconds. If a connection has an empty host name it means that it was not yet retrieved. If the hostname is "." it means that there is not a host name for that IP address, else the string you see is the host name associated with the address.
If you use small refresh intervals you may be able to see netactview host name retrieval connections. They will be udp '53 domain' connections to your DNS server like: 'udp 42803 CLOSED 126.96.36.199 53 domain google-public-dns-a.google.com 441 netactview'.