Text Compare
Paste two blocks of text and see exactly what differs — line by line, word by word, and character by character where that is what changed.
Paste or type into A and B
Lines are matched up first, then a replaced pair is compared word by word and a near-identical word character by character — which is why changing one octet of an address marks the octet, not the line.
The count above the output is the number of changed regions, not changed lines: two lines edited next to each other are one thing to look at and count as one, and long unchanged stretches collapse to three lines either side with a control to open them again.
The text in A and B is never sent anywhere, never written to your browser's storage and never put in the address bar, so there is no link that could carry it — only the checkboxes and the Inline/Side-by-side choice are remembered between visits.
Comparing what a domain publishes rather than two pastes of it? Domain Check