Sunday, September 12, 2010

BSCI - Multicast - GLOP addresses

The GLOP address range, (pronounced GLOP not G-L-O-P), was originally specified in RFC2770 and was an experimental, public, statically assigned multicast address for publishers and ISP's to source content on the internet.

The method of assigning one of these experimental address was called GLOP. Implementers were assigned 255 addresses from the subnet. The actual address assigned was determined from the ASN the implementer already used.

The address assigned set out the values of the 2nd and 3rd octet of the GLOP address. That is to say, all GLOP addresses start 233 (octet #1), the you had octet #2 and #3 to allocate, and finally you knew you had 255 addresses to choose from so the 4th octet was always a value of your choice 1-255.

Octet 2 and Octet 3 were determined by a calculation involving the ASN already assigned to the implementer and therefore, in theory, the GLOP address that resulted was unique (i.e. - not allocated to another organisation).

To determine the value of octet #2 and octet #3 do the following: (example taken from RFC2770)
i)Take the ASN 5662
ii)Convert it to binary and pad the left of the binary figure with zero's until you have 16 bits (octets 2 and 3 combined) - 0001011000011110
iii)Divide the 16 bits in 2 and you are left with 2 octets (8bits each)
iv) convert these to decimal - 00010110 = 22, 00011110 = 30 and add to you GLOP address starting 233.X.X.X
v) GLOP addresses always have /24 subnet mask because the implementer can select 255 addresses to be assigned locally. As a result in this example the GLOP address will be 233.22.30.XXX/24

Further reading:

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