An often-cited benefit of blockchain technology is the transparency it offers. Transactions, blocks, and addresses are publicly auditable, and the tool you use for that job is called the block explorer.

The block explorer is a graphical tool to view and explore data on the blockchain, like how a web browser lets you access the information on the internet.

There is a block explorer available for almost every public blockchain. If you know how to navigate one block explorer, you will know how to navigate all of them. It is quite intuitive and aids in understanding of how a blockchain is structured and how it works.

It allows you to browse the history of a given chain. You can review the transaction history of a given address, the set of transactions in a block, as well as the status of transactions.

Let’s take a look at a real-life example:

Horizen’s Block Explorer

Explorer address summary

This is what you can expect when you are searching for an address with a block explorer. The address that was looked up for this example on a Horizen block explorer is at the top: znf7… Let’s say this is Bob’s address.

On the top of the page, you will find a summary of the addresses activity. The information provided here will include the total amount received and sent from this address, as well as the current balance. The address we are looking at received a total of 42 ZEN. Bob didn’t send any ZEN to other addresses yet, and therefore still has a balance of 42 ZEN.

Explorer address tx

You will find all the transactions that this address was involved in below the address summary. This particular address was only part of a single transaction thus far. Every transaction is characterized by its identifier - the transaction ID - which is the blue string at the top of the gray box. The transaction we are looking at had one input (left) and two outputs (right). The input is what is sent, and the output(s) is what is received.

Let’s say Bob received his funds from Alice. Alice’s address is next to the input on the left, znd3… Alice had an output of 46.6 ZEN in her address, but only wanted to send 42 to Bob. Alice did this by using her entire balance of 46.6 ZEN as an input (left) and creating two outputs: one output of 42 ZEN she wanted to send to Bob, and one with the remaining 4.6 ZEN going back to her own address as change.

Now there are three pieces of information left uncommented:

  • The sender added a transaction fee of 0.0001 ZEN. Transaction fees are a measure to prevent spam. When sending only a few transactions, the fee is negligible. If you were to send large amounts of spam transactions, the cost would quickly add up and become prohibitive.
  • The number of confirmations is a measure of how old a transaction is. Every block that is created after a transaction is included in the blockchain is called a confirmation of that transaction.
  • There is the total volume of the transaction.

Let’s explore the beginning of blockchain technology and have a look at Bitcoin in this next example.

A Bitcoin Block Explorer

Latest blocks

There are a number of block explorers out there, oftentimes several versions for the same blockchain. Since we want to go all the way back to the beginning of cryptocurrencies in this example, we will look at one of the most popular Bitcoin explorers out there.

You will see an overview of the most recently created blocks on the landing page. Let’s go through what we see here:

  • Height: the height is the number the block carries. The very first block created was block #0, after that came block #1 and so on.
  • Age: Is the age of the block. While self-explanatory, there is a piece of interesting information between the lines: you might know that the block time, the interval in which new blocks are mined, is 10 minutes with Bitcoin. How come it took only 6 minutes to mine block 539.772 and 9 minutes for block 539.771? This is because the block time is an average. There will always be blocks created slower and a few that will be produced a lot faster, but on average it will take 10 minutes.
  • Transactions: The number of transactions included in this block.
  • Total Sent: The 1547 transactions that took place add up to almost 4700 Bitcoins sent.
  • Relayed By: The miner or mining pool that solved the block and earned the block reward.
  • Size: The amount of storage the block takes up
  • Weight: A metric that refers to the size of a block. It is a more technical metric but as you can see, the more transactions in a block and the bigger the file size of a block, the bigger the weight.

The Genesis Block

Let’s have a look at a single block. The height of the very first block is 0 as we said before. If we want to look at the very first Bitcoin block, we can use the search function and enter “0”. The very first block of a given chain is usually called the genesis block.

The Genesis Block

The genesis block contains only one transaction. This is the minimum amount you will see since every block has a so-called coinbase transaction, the transaction that is used to issue new bitcoins to the miner that solved the block to reward them for their efforts.

The reward for mining Bitcoin, in the beginning, was 50 BTC. The block reward reduces by half every four years, or more precisely, every 210,000 blocks. This has happened three times so far. A miner today receives 6.25 BTC per block.

Other than the transaction that rewarded the miner, Satoshi Nakamoto himself in this case, there is no transaction in this block, that is why the estimated transaction volume is 0. Today there is a transaction fee attached to most transactions, but this wasn’t always the case, especially when the network was still in its infancy.

We have talked about the height before, so we will get to the timestamp now. The first bitcoin was mined on the 3rd of January 2009, the birthday of the very first cryptocurrency.

The “Relayed By” field shows who mined the block. Today’s mining pools and their reward addresses are mostly known. This is how the block explorer “knows” who mined the block: it compares the address that received the newly generated coins (1A1zP1…) in the coinbase transaction with a number of known addresses from mining pools. In this case, we do know who the miner was, although the block explorer doesn’t tell us: Satoshi Nakamoto himself/herself/themself/themselves.

The difficulty describes the threshold the block hash has to be smaller than or equal to in order to be considered valid. It can be interpreted as the number of leading zeros the block hash must have. As you can see on the right, the genesis block hash had 10 leading zeros. Looking forward to block 539.772, it needed to have at least 18 leading zeros.

Bits, Size, and Weight all refer to the size of the data of a given block, not too spectacular. Version refers to the software that was run at the time the block was created.

Now we get to another interesting information: the Nonce. When miners try to solve a block, what they are doing is putting different values in the Nonce (Number used once) field and calculate the hash of the block. If the result doesn’t have the minimum amount of leading zeros required (most often it will have none at all), they increment the nonce and hash the block again.

This is repeated until a miner finds a nonce that produces a hash below the target. In the case of the very first block, Satoshi’s computer tried more than 2 Billion values (if he started at 0) before he found a value that produced the desired result. He received the very first 50 bitcoin as a block reward for this.

The First Bitcoin Transaction

What you will most likely use a block explorer for is checking a transaction status. You can find a given transaction either by searching for one of the addresses involved, the sending or receiving address, or by the transaction ID (TXID). Most wallets will show you the transaction ID for all of your transactions.

If we want to look at the very first cryptocurrency transaction ever done, we need to go to Block #170, where Satoshi Nakamoto sent Hal Finney 10 BTC.

Bitcoin Transaction

It is the first block on the Bitcoin blockchain that contains more than one transaction. We can find the coinbase transaction on the very top. Below is the first transaction as you would expect it. One person sending coins to another one.

Summary

The block explorer does for the blockchain what a web browser does for the internet - it lets you access the data. The things you are likely to look up in a block explorer are your addresses and your transactions. Additionally, you can open entire blocks and see all transactions contained.

When you send a transaction to a merchant or exchange and don’t get a notification right away about them receiving your funds, it can feel a bit scary. By using the block explorer to look up your transaction, you can verify that it is on its way, and everything worked out.`