Hi,
I understand the IPv6 header is 40 bytes in length - fixed.
One of the fields in the v6 header is Payload length.
This field is fixed at 2-bytes in length and depending on the value of
each bit will determine the total length of the data field. No
problem.

I understand 2 Bytes = 0000 0000 0000 0000 = 64 different combinations
BUT how does this equate to 64KB?

The statement is
"The fact that the payload length field has 2 bytes limits the maximum
packet payload size to 64 KB."
How is it that 64 KB is arrived?
Thanks for any understanding.

DSv6Guy <fsebera@yahoo.com> wrote:
> I understand 2 Bytes = 0000 0000 0000 0000 = 64 different combinations
> BUT how does this equate to 64KB?

It's basic binary number representation. There are 2 possible permutations
in 1 bit. 4 permutations in 2 bits, 8 permutations in 3 bits, 16
permutations in 4 bits, and so on - doubling for each additional bit. One
reaches 64 permutations in only 6 bits. By 16 bits there are 65536
permutations (or 64k, where k = 1024). I'm not sure how you are coming up
with only 64.

On Oct 2, 3:05 pm, DSv6Guy <fseb...@yahoo.com> wrote:
> Hi,
> I understand the IPv6 header is 40 bytes in length - fixed.
> One of the fields in the v6 header is Payload length.
> This field is fixed at 2-bytes in length and depending on the value of
> each bit will determine the total length of the data field. No
> problem.

Homework is such a drag.

Still, always go to the source. In this case, RFC 2460 is the source.
It's free, as are all RFCs.

http://www.ietf.org/rfc/rfc2460.txt?number=2460

Actually, the header is variable in IPv6 (and IPv4), in the sense that
you can add extensions to it. But for the purposes of the "length"
field, it is assumed to be a fixed 40 bytes, as you said. This is
different from IPv4, where the length of the header is also specified
as part of the header.

RFC 2460 says:

Version 4-bit Internet Protocol version number = 6.

Traffic Class 8-bit traffic class field. See section 7.

Flow Label 20-bit flow label. See section 6.

Payload Length 16-bit unsigned integer. Length of the IPv6
payload, i.e., the rest of the packet
following
this IPv6 header, in octets. (Note that any
extension headers [section 4] present are
considered part of the payload, i.e., included
in the length count.)

Next Header 8-bit selector. Identifies the type of header
immediately following the IPv6 header. Uses
the
same values as the IPv4 Protocol field
[RFC-1700
et seq.].

Hop Limit 8-bit unsigned integer. Decremented by 1 by
each node that forwards the packet. The packet
is discarded if Hop Limit is decremented to
zero.

And the 16-byte source and 16-byte destination addresses. Total is 40
bytes.

The "next header," if one or more exist, would have the length of that
extension in it, so you can always tell where the payload begins.

> I understand 2 Bytes = 0000 0000 0000 0000 = 64 different combinations
> BUT how does this equate to 64KB?

2 bytes is a series of 16 bits. Each one of these can be set to 0 or
1. So how many combinations can you have?

2^16 = 65536 = 64K

Bert

On Oct 3, 3:05 am, DSv6Guy <fseb...@yahoo.com> wrote:
> Hi,
> I understand the IPv6 header is 40 bytes in length - fixed.
> One of the fields in the v6 header is Payload length.
> This field is fixed at 2-bytes in length and depending on the value of
> each bit will determine the total length of the data field. No
> problem.
>
> I understand 2 Bytes = 0000 0000 0000 0000 = 64 different combinations

Nothing to do whth 64 combinations. Infact you'll find you can
potentially have more than 64 THOUSAND combinations with those bits.
Didn't anyone teach you maths?

> BUT how does this equate to 64KB?
>

I wonder what is the decimal value of the binary number
1111111111111111 ?