If you own NFT, have you ever wondered what you are actually owning? You could say a jpeg but it's actually not correct.
NFTs are non-fungible tokens: each one different from the next. You think about BTC, my 0.50 BTC will equal your 0.50 BTC. If, on the other hand, I own a Bored Ape (NFT), it will be different from the Bored Ape owned by another person.
Ok, I don't want to change the concept that NFT=Jpeg, however a distinction should be made between:
1) Erc721 token + Metadata (Jpeg + attributes) registered directly on the blockchain
2) Erc721 tokens in blockchain, Metadata in centralized servers
If you hold an NFT in your non-custodial wallet (Metamask, Phantom, Keplr, etc) you certainly have the Erc721 token in your wallet! You are the owner and you can do whatever you want with it. However, for the majority of cases, the photo you see (Jpeg) is not registered in the blockchain but on centralized servers. What does this mean? If the centralized server deletes that photo, your NFT will always remain in your wallet but you won't see the photo again! Why aren't the photos recorded directly in the blockchain? You have to consider that on the Ethereum blockchain, the most valuable thing is the space on the block to add transactions. The heavier a transaction is, the more cumbersome and expensive it will be! Imagine having to save hundreds of thousands of photos on the blockchain... how do they solve this problem? On-chain a link is inserted and this link refers to centralized services where the photo is saved for the NFT number 2567 or for that number 8799.
CHECK WHERE THE METADATA IS SAVED
Trivially you can go to Opensea, search for your collection of interest and then click on the collection's Etherscan link at the top right. Later in Etherscan you will read about the characteristics of the collection. If you go to "Read Contract", for many collections you will read a reference to the IPFS site (many collections on Ethereum use this service).
What is IPFS? Their page reads "A peer-to-peer hypermedia protocol designed to preserve and grow humanity's knowledge by making the web upgradeable, resilient, and more open". Oh well, a P2P service, it's not really a centralized service however photos and attributes are hosted up here and not on the Ethereum blockchain (where there is a link to this site). For example here is the link to the Jpeg of a Bored Ape:
The same site is also used by the Azuki collection which has had great success (floor: 6 ETH):
For example the Trump collection, save Jpeg and attributes on a centralized site. These are the steps you need to take to check where the photo is saved:
1) Find the collection you are interested in on Opensea
2) Find the number of mints of an NFT (random) in the collection
3) At the top right, click on the link that refers to the explorer (Etherscan, Polygonscan etc)
4) Press on contract
5) Read contract
6) Find TokenURI
7) Enter the serial number of the NFT (for example: 5538)
8) Click on the link
9) You will get a page where you can find the link where the photo was uploaded and its attributes
For Cryptopunks initially only the hash was present on the blockchain but in a 2021 update they are actually hosted on the blockchain so they can be considered NFT really registered on Ethereum!
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