What does the Next Header field do in IPv6?

Next Header (8 bits) identifies what follows the current header — either an upper-layer protocol like TCP/UDP, or the first extension header in the chain.

* The Next-Header field chains extension headers to the upper-layer protocol. *

It plays the role of IPv4's "Protocol" field but is more powerful because it also stitches the extension-header chain together:

• If the payload is TCP → Next Header = 6
• If the payload is UDP → Next Header = 17
• If an extension header follows → Next Header = that header's type number (e.g. 44 = Fragment)

Each extension header in turn has its own Next Header field, so a packet can carry a sequence like IPv6 → Routing(43) → Destination Options(60) → TCP(6).

Tip: To find the actual payload protocol, you "walk the chain" of Next Header values until you hit a non-extension protocol number (or 59 = No Next Header).

Go deeper:

• IPv6 packet — Next Header (Wikipedia) — how Next Header both names the upper-layer protocol and stitches the extension-header chain.