Poll tutorial: running the command 'python manage.py runserver' produces an error

Hello

When I run the initially created project using the command 'python manage.py runserver' gives me 

Error: [Errno 10013]

When I run this command though 'python manage.py runserver 8001', then I can see the site on http://localhost:8001

Can you tell me why this happens and what are those 4 digit numbers after localhost?

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