gistfile1.rb

# This method is heavily adapted from the Rails method of determining the subdomain.
require 'rubygems'
require 'sinatra'

helpers do
  def subdomains(tld_len=1) # we set tld_len to 1, use 2 for co.uk or similar
    @subdomains ||= if (request.host.nil? ||
                        /\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/.match(request.host))
                      request.host.split('.')[0...(1 - tld_len - 2)]
                    else
                      []
                    end
  end
end

get '/' do
  subdomains.join("<br/>")
end