How to Add Arbitrum to Metamask

Arbitrum is an Optimistic Rollup and has made its name one of the top Layer 2 solutions on Ethereum. This section will include ways to add Arbitrum Mainnet and Testnet as your custom MetaMask RPC node

Overview 
1.​Mainnet vs. Testnet (Rinkeby)​
2.​Create a free Alchemy account​
3.​Create an Arbitrum App  
4.​Add to Wallet 
​Updating your Metamask RPC manually​
1.​Copy your HTTP API Key​
2.​Update Arbitrum Mainnet​
3.​Update Arbitrum Testnet (Rinkeby) 
Mainnet vs. Rinkeby Testnet
There are two networks on Arbitrum: Mainnet and Rinkeby Testnet. The endpoints are as follows: Mainnet: https://arb-mainnet.g.alchemy.com/v2/your-api-key Rinkeby: https://arb-rinkeby.g.alchemy.com/v2/your-api-key 
Set up a free Alchemy account
This is a very important step. Just do it. Here.
Since Arbitrum is a Layer 2 solution, be sure to select the Ethereum + L2 Ecosystem during sign up.
Create an Arbitrum App
Check out these instructions for creating a new app. Make sure to specify Arbitrum as the chain and choose Mainnet or Rinkeby. 
Add to Wallet 
Alchemy provides an Add To Wallet button for Arbitrum apps. 
This is a more convenient way to add the Arbitrum network your Metamask wallet rather than manually configuring it.
Add to Wallet Button
That's it! You should see your Metamask work a lot better now 😉
​
Updating your Metamask RPC Manually
The below instructions are for updating your Metamask RPC manually instead of using the "Add to Wallet" button. 
Copy your HTTP API Key
After creating your app, you should be able to see your Alchemy app’s chain matches the chain you want to connect to.
Here, you can find your HTTP which you will use for your RPC URL in Metamask. 
Alchemy API Key
Updating Arbitrum Mainnet 
1. Open Metamask and click the network name located at the top-right of the window
2. Click the Add Network button at the bottom of the pop-up window
3. Enter the following network details and click Save
Network Name: Arbitrum
You can name your Network Name anything to remind you which chain you’re connecting to.
New RPC URL: https://arb-mainnet.g.alchemy.com/v2/your-api-key
For the New RPC URL field, grab the HTTP from your Alchemy Dashboard. If you don't have a dashboard account, go back up to the "Set up a free Alchemy account" step and set up your account.
Chain ID: 42161
Currency Symbol: AETH
Optional 
Block Explorer URL: https://arbiscan.io/
Optional 
After clicking Save, you will be directly switched to Arbitrum’s Network within the dropdown list.
That's it!  You've now added the Arbitrum network with Alchemy connected.
Update Arbitrum Testnet (Rinkeby) 
Arbitrum currently has one testnet connected to Ethereum's Rinkeby testnet. This operates exactly the same as Arbitrum mainnet but is built as a smart contract on Rinkeby instead of on Ethereum mainnet.
Adding the testnet is the same process as adding the mainnet shown above, but with a different RPC endpoint. 
1. First, make sure you’ve enabled the Show test networks toggle. 
This will list all the default test networks within your network list. You can find this in Settings>Advanced>Show test Networks
2. Enter the following network details and click Save
Network Name: Arbitrum Testnet
New RPC URL: https://arb-rinkeby.g.alchemy.com/v2/your-api-key
Chain ID: 421611
Currency Symbol: AETH
Block Explorer URL: https://testnet.arbiscan.io/ 