Sherlock Holmes lives at the address of 221B Baker Street, London. At the time The Adventure of the Speckled Band was written, this address did not exist. In fact, in Doyle's time, there were only buildings numbered up to 100 on Baker Street. It was precisely for this reason that Doyle chose a higher number-- an address that didn't exist but was based in reality helped to ground his works of fiction in the real London.
The address of 221B Baker Street has since been built and today serves as a museum to Doyle's work and his most famous character: Mr. Sherlock Holmes. Technically, there still is no 221B Baker Street. The museum is housed in the address of number 239 Baker Street, but it has been granted special permission to be listed as number 221.