Pokémon Red and Blue/Route 5-6

'After your visit with Bill, return to Cerulean City and go to the house in the northeast corner. Walk through the backyard to find a path leading to Route 5. The road that goes through Saffron City is closed, but you can take the detour along the Underground Path. Stop off at the Pokémon Daycare before you head underground.'

The blocked path
You'll need to jump down the ledges from Cerulean City if you want to get to the Day Care Center, where you can drop off Pokémon to be leveled up while you walk (they gain EXP at the rate of 1 point for each step you take). Unfortunately, the head of the Center makes all the decisions, so if your Pokémon is full on skills, he may delete a skill you like or abandon the new one altogether, so be cautious. But if you bought that, dump it here and return later for a. You will be charged 100 for each level gained, though.

The gate to Saffron City is closed, so you'll have to take the underground tunnel. Access the tunnel through the building along the right side of the route. Press the A button rapidly as you walk to pick up some randomly-placed items (nothing too exciting; X Special and Full Restore). The Full Restore is exactly 2 squares West of the stairs at the North end of the path. At the Route 5 entrance, a woman will offer to trade you a male Nidoran for a female in Blue or Red, or a for a  in Yellow. Not a great deal in Blue, in which female Nidorans are more common anyway, but Red players should take this opportunity for a rare Pokémon. Also, since Machoke evolves through trade, trading away your Cubone is the only way to get Machamp without a game link cable. You may not have the Pokémon that she wants, but not to worry. No matter how long it takes you to find the proper Pokémon, she will wait patiently.

There are lots of powerful trainers on the other side, but you can avoid almost all of them.

On to Vermillion City
The Underground Path runs south, tunneling beneath Saffron City and emerging at the northern end of Route #6. Since the entrance on this side of Saffron City is also blocked by another thirsty guard, you should now head south to Vermilion City. The path is packed with wild Pokémon and trainers, all ready to challenge your Pokémon to a good battle.