How does metamask know a contract is for ropsten or for main network?

Question

Im using this code to detect for metamask.

Now im using ropsten but i want to know how does metamsk know that my contract only works with ropsten?

window.addEventListener('load', async () => {
    // Modern dapp browsers...
    if (window.ethereum) {
        window.web3 = new Web3(ethereum);
        try {
            await ethereum.enable();
            var accounts= await web3.eth.getAccounts();
            var option={from: accounts[0] };
            var contract = new web3.eth.Contract(abi,address);
            const agiContract = new web3.eth.Contract(agiabi,agiContractAddress);

            const balance = await contract.methods.balanceOf(accounts[0]).call();
            const myDivs = await contract.methods.dividendsOf(accounts[0]).call();


            var supply = await contract.methods.totalSupply().call();
            var agiBalance = await contract.methods.totalAgiBalance().call();

            const agiAccountBalance = await agiContract.methods.balanceOf(accounts[0]).call();



            document.getElementById('agiAvailable').innerHTML = agiAccountBalance/100000000;
            document.getElementById('agiContractBalance').innerHTML = agiBalance/100000000 + ' AGI';
            document.getElementById('contractBalanceSnet').innerHTML = supply/100000000 + ' SNET';
            document.getElementById('snet-holding').innerHTML = balance/100000000;
            document.getElementById('myDividends').innerHTML = myDivs/100000000;
            document.getElementById('wallet').innerHTML = accounts;


            ethereum.on('accountsChanged', async (accounts) => {
              await ethereum.enable();
              var accounts= await web3.eth.getAccounts();
              var option={from: accounts[0] };
              var contract = new web3.eth.Contract(abi,address);
              const agiContract = new web3.eth.Contract(agiabi,agiContractAddress);

              const balance = await contract.methods.balanceOf(accounts[0]).call();
              const myDivs = await contract.methods.dividendsOf(accounts[0]).call();


              var supply = await contract.methods.totalSupply().call();
              var agiBalance = await contract.methods.totalAgiBalance().call();

              const agiAccountBalance = await agiContract.methods.balanceOf(accounts[0]).call();



              document.getElementById('agiAvailable').innerHTML = agiAccountBalance/100000000;
              document.getElementById('agiContractBalance').innerHTML = agiBalance/100000000;
              document.getElementById('contractBalanceSnet').innerHTML = supply/100000000;
              document.getElementById('snet-holding').innerHTML = balance/100000000;
              document.getElementById('myDividends').innerHTML = myDivs/100000000;
              document.getElementById('wallet').innerHTML = accounts;
            })
        } catch (error) {
            // User denied account access...
            console.log(error)
        }
    }
    // Legacy dapp browsers...
    else if (window.web3) {
        window.web3 = new Web3(web3.currentProvider);
        // Acccounts always exposed
        web3.eth.sendTransaction({/* ... */});
    }
    // Non-dapp browsers...
    else {
        document.getElementById('wallet').innerHTML = "Use Metamask on a Desktop or use TrustWallet on a mobile";
        document.getElementById('agiAvailable').innerHTML = "<a href='https://binance.com/' style='color:green'>BUY</a> ";
    }
});

Answer 1

var contract = new web3.eth.Contract(abi,address);
const agiContract = new web3.eth.Contract(agiabi,agiContractAddress);

Any on-chain function-call via contract can complete successfully only if a contract which implements that function is deployed at address.

Any on-chain function-call via agiContract can complete successfully only if a contract which implements that function is deployed at agiContractAddress.

This means that your program may run successfully on one network but fail on another.

MetaMask is not the reason here; your input addresses are.

Answer 2

Metamask doesn't know that your contract only exists in Ropsten. When you log into metamask, you select the network that you want to work with in the dropdown in the picture below.

This is the network that web3 will be interacting with. You can check the network that you are using by looking at the network id of the web3 object. You can do this with the code below. If your contract only works with Ropsten, maybe you'd want to present a warning or suggestion to change networks if the user isn't on Ropsten.

web3.version.getNetwork((err, netId) => {
  switch (netId) {
    case "1":
      console.log('This is mainnet')
      break
    case "2":
      console.log('This is the deprecated Morden test network.')
      break
    case "3":
      console.log('This is the ropsten test network.')
      break
    default:
      console.log('This is an unknown network.')
  }
})

Code from this answer