IP Address is used for network devices identification in the IP network. Let’s have a look at it like it will be a traditional post. We are buying an envelope (carrier), writing a letter (content), place the content into this envelope (addressing for the recipient) and we bring this envelope to the post office (the media). We hope that this letter will reach its destination then. Additionally we place sender details on the back of this envelope in case of something was wrong and the letter can’t be delivered to its recipient. It gives a possibility to return this letter to the sender.
This is very similar to the idea of IP network in case of idea. The difference is in data representation (letter vs. digital message) and the time the message need to reach its destination (days vs. miliseconds). IP network is a media where data packages are sent. Size of the packages are adjusted to the media character.
Let’s say we have written a message in e-mail software like Outlook or Thunderbird. This letter contains of couple a4 size pages. To send this letter, apropriate engines have to split it into smaller pieces, pack into network packets and send to the recipient through the network. When the recipient receive these pieces, he have to reassemble them into the complete letter. So he need to know what was the the splitting order. This is done using sequence number which is attached to every network packet.
So every packet has its source IP address (sender address), destination IP address (recipient address) and additional information which is needed to make transmission beetween sender and recipient. If there will be two equal IP addresses in the network, then transmission errors will occure. So the IP addresses should be unique.