Bitcoin Forum

Here we go with another merit giveaway thread. Once again the awards are to be given to new and junior members only. The thread will be self-moderated, and when one topic is answered, I'll add another topic. Spamming and very low value answers will be deleted. All replies must be original, and not copied from other sources. If you don't know the answer, then Google the question and read about it. When you think you understand the topic, then post the answer in your own words. So here is the first question.

The Bitcoin blockchain is constructed from a sequence of linked blocks. Please explain how the blocks are linked

Thread restarted because i forgot to make it self-moderated. The merit awards will be in the range of 1 - 5 based on the comprehensiveness ofyour answers.

If I understood the question right, answer will be: each new block has the hash of previous one within. What is hash? As I understand it, without all functions and math stuff, it`s a big set of symbols, which is calculated based on such set of previous block and special algorithm. So hash is the key.

I gave you a merit for that post. It's a pretty generalised basic explanation, so there is still plenty of scope for members to expand on it.

Each block carries a specific quantity of transactions and a hyperlink to the preceding block. This is the thing that connected one block after the others in time. Blocks are therefore organized right and fully secure with those SHA / HASH ( Algorithm Stuff ).

Every node can group transactions collectively right into a block and broadcast it to the network that offers the name to the entire machine ultimate system known as blockchain.

Block is a field 80 byte. The following section of information is the number of transactions in the blocks and the primary transaction.
A common transaction on 250 bytes option to the type of the multi transaction transactions to be used to be more than that the requirements of the standard - you can see more about other transaction types here here. A average block contains the 500 transaction transactions - equivalent with less a transaction / seconds.

Thanks a lot  :) Wasn`t sure that all technical detailes needed, so I tried to simlify. Should I make an answer deeper or allowing others to participate will be a better call?
I think, establishing/explaining some rules in first post may be appropriate. For exaple, can a person answer one question multiple times and if yes - which amount of  posts is allowed etc.

Here is a bit of clarification then. The rules -

- No spam
- No plagiarism.
 - Newbies and beginners only
- You can correct previous posts if you think they are wrong - for example, miners are the guys who insert transactions into blocks
- Please stay on topic - this question is about block linking, and not about block composition in general
- You can make more than one post, but all posts must be beneficial for the board, and helpful for new Bitcoiners.

I'll try to add a little bit on that answer.
As it was previously mentioned, a new mined block contains, within its header, the hash of the precedent block.

If we look closer to the block, we can also find, still within the header, Merkle tree root.
Basically, a Merkle tree is a binary tree, whose lowest level nodes are transactions, and whose intermediary level nodes are, either sum hashes of intermdiary nodes, either transactions hashes.

Basically, a BTC transaction is made of 2 transactions: the one that is required by the spender, and what is called the UTXO (Unspent Transaction Output).
An UTXO is what is left to someone who spent BTC, and every BTC nodes keeps a copy of all the UTXO.

So we have seen here that basically, BTC blocks are either linked together by containing previous block hash, but they are either linked together at the level of transactions, and more precisely at the level of UTXOs.

I'm a bit of a noob to blockchain in general but I'll do my best to simplify it for those who are just starting out in this wonderland.

The blockchain is at its most basic form, a ledger. This ledger keeps track of transactions which are linked together (the blocks).
Every change to the ledger (incoming or outgoing crypto transactions) is signed to prove that the person actually owns the coins.
This ledger is also stored on every miner's computer so there can be no discrepancies. The beauty of the system is no-one needs to know the identity of where their transaction went. A person withdraws btc from their wallet, the transaction is sent onto the blockchain, and a short period of time later, it arrives at the destination. The allure is the decentralized nature of the beast, no banks to pass throughor looking up SWIFT codes, days waiting for funds to be accepted if in another country, it's virtually anonymous.

Hope this clears it up a bit, good luck on starting out on this journey, sometimes I really do feel like Alice in Wonderland, the potential for Blockchain is just starting to be realized but it's definitely a future prospect for many companies.

As the first guy that answered your question, the blocks are linked together by the hash of the previous one (parents hash) and the newest one to the first ever created (the genesis block). Merkle trees are used here (just like Ethereum) to store the transaction stuff into a block.


Don't ask me to go on a tech explanation with a lot of complicated words or terms ;)

The last node of the block contains the address of the next block. So at the end of the node the block checks the value and jumps to the address of the next block which has been mentioned.
Maybe i draw a small example ?

I think each block contains a hash of its previous block. In bitcoin blockchain, the block header has a field for previous block hash. Hence, all blocks will contain a reference of its previous block, and this could build up a chain of blocks.

So. To understand how blocks in Bitcoin`s blockchain are linked, we should look into a structure of block itself. Let`s imagine, that each block is a text document with a set of fields. Now, we can say, that each block consists of 5 components (5 fields) :
Magic number (value always 0xD9B4BEF9)
Blocksize        (number of bytes following up to end of block)
Blockheader  (we`ll look into it below in details)
Transaction counter (a number)
Transactions (the list of transactions)
In our case, we are interested in 2 of them (except Blocksize, Transaction counter (I just don`t know what it`s used for ;D) and Magic number (last one is constant, as far as I know)). Though the most important magic lives in blockheader, we can say that list of transactions are making connection between blocks too (mostly, because one of the key fields of header is formed on the ground of this list). Also list of transaction contains info about who had an honor to generate this block, info about transaction fee etc. and a little bit of stuff, nitrocryptonitro was writing above. It is very important field.
Let`s talk about header now. It contains of 6 fields:
Version             (Block version number. it`s about software, don`t really interesting for us now)
hashPrevBlock   (hash of the previous block header)
hashMerkleRoot (hash based on a list of block`s transactions)
Time                 (time of generating)
Bits                  
Nonce               (number, which value is set in a way, that hash of the block will contain a run of a leading zeros. As I understand, calculating this one is one of the most important things that miners do)
Anyway, we are more interested in those, painted red. As I understand, hashMerkleRoot of block #1 is becoming hashPrevBlock for the next one (#2) and it goes on and on. So, that`s how all blocks are chained one to another.
There is also Height of the block or it`s ordinal number. You can check current block`s Height here http://blockexplorer.com/q/getblockcount (http://blockexplorer.com/q/getblockcount). Height of the block is a number of blocks, that was generated before it.
Now, lets explain what a hash is a little bit, just so you can understand what does this word mean. In simple words, it`s a function that can take an input data of any size and with the help of mathematical operations return output data of a fixed size. It`s something like ZIP archives in world of math, if you remember those. Imagine, how much info a list of transactions contains, and you`ll undestand why Bitcoin needs a special tools to ZIP it.
Hope It`ll help somebody to understand a little bit this complicated stuff.
P.S.I`m not an expert in this matters, so feel free to fix my mistakes if you find any.

There's something I want to add, but I'm almost certain I will have dificulties to explain it.

The question is "explain how the blocks are linked".

That made me ask to myself that other question: Explain how certain blocks aren't linked together.

That last question leads to the matter of multiple nodes mining a different block at the same time and spreading their own block in the BTC network.

Only one can be used by the network, otherwise, that would be a case of double-spending (when someone can spend twice its BTC because there are 2 versions of the blockchain tail).

To solve that problem, BTC blockchain is designed to choose the longest blockchain available.

In a way, we can say that this characteristic of BTC blockchain allows certain blocks to be linked together, and other ones to be discarded.

Am I off the subject, or not? I'm not completely sure...

I wondered if somebody would introduce orphan blocks. :)

It's become quite an interesting discussion, and I'll reread the posts shortly. I'm also thinking of opening the thread to senior members once the post count has passed 50.

Orphaned blocks are valid blocks which are not part of the main chain. They can occur naturally when two miners produce blocks at similar times or they can be caused by an attacker (with enough hashing power) attempting to reverse transactions.

OK, now I`m tempted to write about 51% attack and why one big mining pool sometimes need to split :)

I think "hash of the previous block" and "hash of the block" is the key two elements when we talking about how the blocks are linked in a blockchain. Every block in a block chain has a hash. It's like a finger print. It identify the block and all it's content and it's always unique just as like fingerprint. Once a block is created it's hash is calculated. Changing something(data) inside the block will cause hash to change, in other other words we can say hashes are very useful when you want to detect changes in the block. If the fingerprint of a block changes, it no longer the same block. Another important element of a block is the hash of previous block. This is what effectively creates the chain of block. I think following figure will explain more clearly what I have explained.( Actually that's what I got understand regarding linking of blocks of blockchain)

Blocks are linked by many characteristics, some unique, some not: time, transactions details, hash, signature details, block height...
Blockchain allows the transactions to be validated across the Bitcoin network
through a peer to peer type of involvement - meaning any individual can participate in the validation of these transactions.
To participate in this p2p and support the blocks, there are conditions: one needs to have enough electricity and computational power to resolve the complex calculations that
will validate the signatures of the transaction (secure hash algorithm).
Once validated, is sent to the other peers who will take it from there and make new calculations.
After everything is validated and transaction sent across the network, a new block was added to the blockchain.
Blocks are linked by many complex features, not sure if most of the miners can explain in detail how blocks are linked, I for sure cannot.

As far as I understand the blocks are linked through the previous hash block that is saved on the block header. The block header has a unique identifier called the block header hash, and inside the block header we can find three main components. The three components of block header as follows:

• Previous Block Hash (which means that the hash will be linked through the block header because the previous block is one of the blockheader components)

Each Previous Block Hash has only one parent to identify.
Let me discuss it in simple way the IDENTITY of the father will be embedded on the child’s IDENTITY to the next child and so on.

• Timestamp, Difficulty and Nonce (This is for mining)

• Merkle Tree Root
Since the header hash is not stored on block structure and then it is obtained by double hashing the block header by using algorithm and that is SHA256 algorithm used for bitcoin since there are other popular algorithm such as SCRYPT.

This seems so hard to answer but i'll try. It is like a family where the next hash block is identified by the identity of the previous block and the next block will also have same identity as the previous hash block. The linked hash blocks are like bloodline where it start from the parent to the son to the grandson and to the next generation and that's how the hash block linked to the previous hash block with the use of math algorithm and the algorithm used in bitcoin is SHA-256 mentioned on your previous thread about best popular algorithm.

Hello everyone! Here are my thoughts about this question.
Blocks are linked one with another by containing an information about previous one. As guys above have written, it containes in block`s header (according to the Bitcoin Wiki, too) in a form of hash. I can only add, that a "nonce" is a number, that everyone is looking for solving that mathematical puzzle. And a first one to find it will get a prize and here comes the new block. This mathematical puzzle, that I mentioned earlier may have several resolutions and as we know, a great amount of computers are working on it. So, sometimes it happens that couple of participants are finding different results that are matching. So they begin to build following blocks based on previous results. And there are 2 versions of a blockchain in that moment of time. It lasts till one of chains become "longer" for some reason. Then all blockchain refreshes and "longer" chain become "true" one (ak "main"). Usually it takes around 3 blocks to identify which way blockchain will go. That`s why a conception of "confirmation" exists. One confarmation is one block built at the top of one, that contains details of your transaction. So where do block of "shorter" blockchain goes? Nowhere. They are just abandoned. That`s why that one`s are often called "orphaned blocks" - valid blocks, that are not a part of a main Bitcoin blockchain.

Thanks for all the replies, there is an interesting mix of ideas and opinions in them.

I'll open this thread for anyone to post replies. It will be interesting to see the comments from senior members.

each block contains a reference to the previous block, that's why it's called "chain"

reference: https://www.blockchain.com/en/btc/orphaned-blocks

This seems to be a good point to list the contents of the block header, or maybe not, it looks terrible if I copy and paste.

You can find it on this page.

https://bitcoin.org/en/developer-reference#block-headers

when a header is classified as alternative in cardano blockchain?