All I know is channel 21 on my TV is Comcast Philly.
There's the Comcast Network (CN8) and Comcast SportsNet Philly - is it CN8 perchance?
Edit: Checked tvguide.com for zipcode 20002 -chose Comcast Washington - it's CN8 programming.
I get both in PA - CSN Philly is Phillies baseball, 76ers, flyers, etc.