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Posted on 2008-10-02
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Since 248.255.255.255  255.255.255.255 is not used as part of a class E address does anyone know what it is reserved for ?
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Question by:jbovalley
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Expert Comment

by:Mysidia
ID: 22630513
240.0.0.0/4  is reserved by IANA for special use, possibly in the future.
See RFC 3330
Or the new online database listing:
http://www.iana.org/assignments/ipv4-address-space/

Just because it's not allocated  and not part of the multicast or former 'class E' blocks  does not mean its function has already been defined.
Some of these already have special defined functions like 255.255.255.255
[limited broadcast]

This address space is unlikely to ever be assigned; although in theory,
IANA could choose to allocate it to IP registries RIRs, or other those ranges may
in the future be allocated for some other purpose.

For example, in the face of massive NAT, it may be beneficial to have
additional private ranges.

The RFC1918 space can sometimes be too small.

The IETF may invent new communication technologies that require new types of
IPv4 IPs, and IANA may allocate some blocks from the 'reserved for future use'
range.

But as time passes, and with future protocols and enhancements likely to be directed towards IPv6, it seems unlikely, except when implementing IPv4 backwards
compatibilities in newer IP-based protocols.



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Expert Comment

by:urgoll
ID: 22630516
Class E is (or was) defined as 240.0.0.0/4, or the range from 240.0.0.0 to 255.255.255.254.  This range is no longer called 'Class E', but is still reserved for future/experimental use by ICANN.

There are multiple proposals to change their status, but nothing conclusive yet:

http://tools.ietf.org/html/draft-wilson-class-e-02
http://tools.ietf.org/html/draft-fuller-240space-02
http://tools.ietf.org/html/draft-savolainen-indicating-240-addresses-00
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Author Comment

by:jbovalley
ID: 22630752
ok maybe I didn't state my question correctly....what I ment to ask is....
if Class E is reserved as 240.255.255.255 - 255.255.255.255 then why is the first bits reserver as 11110 and not 1111... it would seem to me that class E is reserved as 240.255.255.255  through 247.255.255.255
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Expert Comment

by:kdearing
ID: 22630961
???
"if Class E is reserved as 240.255.255.255 - 255.255.255.255"
Where are you getting this?
The reserved range is 240.0.0.0/4 (formerly called Class E)
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Expert Comment

by:Chris Dent
ID: 22632230

> Where are you getting this?

Seconded...

> why is the first bits reserver as 11110 and not 1111

The reserved bits for Class E are already 1111. If you try to reserve more you don't have a contiguous range unless you're operating from 248 and above only (with 11111xxx).

Class E is this:

Network:    11110000 . 00000000 . 00000000 . 00000000 (240.0.0.0)
Mask:         11110000 . 00000000 . 00000000 . 00000000 (240.0.0.0)
Broadcast: 11111111 . 11111111 . 11111111 . 11111111 (255.255.255.255)

Chris
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Author Comment

by:jbovalley
ID: 22632721
mybe I was  misinformed...I was told t by a teacher that Class Ewas defined as 240.0 0.0 - 255.255.255.255.
also that the fist bits in the class E was 1111, however , what is actual used as class E is
240. through 247. And from this I assumed that the first bits would then be 11110. Is ther any truth to actual usage to the class E being from 240 to 247 .
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Accepted Solution

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Chris Dent earned 2000 total points
ID: 22632866

> Is ther any truth to actual usage to the class E being from 240 to 247 .

Hmm, I've never seen a statement like that before, it's not documented in the original RFCs or by IANA and we would have to treat those as authoritative sources unless another published source comes to light.

You have the links for Wilson, Fuller and Savolainen's requests to reclassify the range above which is about as near as we get to anything authoritative no alternate use.

> And from this I assumed that the first bits would then be 11110

Which would be correct if it were split like that :)

That zero represents 8. Having it set to 0 would limit you to a /5 (248.0.0.0) mask. Splitting the defined subnet in half and giving this:

240.0.0.0 to 247.255.255.255 (Reserved bits: 11110)
248.0.0.0 to 255.255.255.255 (Reserved bits: 11111)

Bear in mind that at the end of the day we're better going with the list above from IANA. The old classful designations aren't used so heavily since CIDR was drafted in RFC 1518 and 1519.

Chris
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