The first thing investors look for is a Standard Industry Classification (SIC) code. SIC codes are a system designed by the US Government to classify businesses. Unfortunately, ISPs do not fit precisely into the SIC system. I use SIC code 7375, "Information Retrieval Services" to classify ISPs. MindSpring, on the other hand, uses code 7389, "Business Services, Not Elswhere Classified." Your best bet is to use SIC Code 7375, or visit OSHA's SIC Search for a complete list of SIC codes.
This is a tad-bit late, but just FYI SIC Codes are now officially outdated. A lot of institutions (Banks, Investment Houses, Vendors etc.) have not made the official switch themselves, but everyone should now take into consideration what their NAICS (North American Industry Classification System) Code is. I believe there is a conversion (SIC > NAICS and NAICS > SIC) table online, so that will be a first start.
We've recently stopped using SIC Codes to identify industries and now use NAICS both internally (our company profile, filings, trade references etc) and externally (IDing client industries when opening accounts with us).