The domain name 1e100.net is a Google-owned domain name used to identify the servers in their network.
According to Google back in October 2009 they started using a single domain name to identify the servers across all Google products, rather than use different product domains such as youtube.com, blogger.com, and google.com.
If you look at http://whois.domaintools.com/1e100.net this shows the domain belongs to Google, and was registered in 2009.
If you are looking at firewall or proxy server logs you may see this quite often, even if the PC has no user activity.
To see a complete list of Google domain names click here.