All 0's and all 1's subnets...availables ??? - TCP-IP

This is a discussion on All 0's and all 1's subnets...availables ??? - TCP-IP ; Hi. I would like more info on the following: I want to make some calculus of subnetting and I am looking for info about the numbers of subnets available. RFC 950 says I should use n bits for addressing till ...

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  1. All 0's and all 1's subnets...availables ???

    Hi. I would like more info on the following: I want to make some
    calculus of subnetting and I am looking for info about the numbers of
    subnets available.
    RFC 950 says I should use n bits for addressing till 2^n -2 subnets, and
    I should avoid all 0's and all 1's subnets.
    But on "VLSM" RFC 1878 it says it is not necessary to avoid these
    subnets. What should I do ? What is current RFC being applied ???

    If I dont make VLSM (ie. all my subnets have the same number of hosts) ,
    could I use all 0's and all 1's subnets ? Or are reserved ?

    Thanks
    Alberto

  2. Re: All 0's and all 1's subnets...availables ???

    Alberto wrote:
    > Hi. I would like more info on the following: I want to make some
    > calculus of subnetting and I am looking for info about the numbers of
    > subnets available.
    > RFC 950 says I should use n bits for addressing till 2^n -2 subnets, and
    > I should avoid all 0's and all 1's subnets.
    > But on "VLSM" RFC 1878 it says it is not necessary to avoid these
    > subnets. What should I do ? What is current RFC being applied ???


    I suspect that as you go up in RFC number you will see which supercede
    which previous ones. In this case 950 and the concept of subnet
    address has been superceded by (IIRC) CIDR - "class-less" IP
    addresses.

    > If I dont make VLSM (ie. all my subnets have the same number of hosts) ,
    > could I use all 0's and all 1's subnets ? Or are reserved ?


    Only if your hosts are running really old software that isn't up on
    the current RFCs.

    rick jones
    --
    The computing industry isn't as much a game of "Follow The Leader" as
    it is one of "Ring Around the Rosy" or perhaps "Duck Duck Goose."
    - Rick Jones
    these opinions are mine, all mine; HP might not want them anyway...
    feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

  3. Re: All 0's and all 1's subnets...availables ???

    In article <_Whig.2328624$kp.12532393@telenews.teleline.es>, Alberto writes:
    > Hi. I would like more info on the following: I want to make some
    > calculus of subnetting and I am looking for info about the numbers of
    > subnets available.
    > RFC 950 says I should use n bits for addressing till 2^n -2 subnets, and
    > I should avoid all 0's and all 1's subnets.
    > But on "VLSM" RFC 1878 it says it is not necessary to avoid these
    > subnets. What should I do ? What is current RFC being applied ???


    RFC 1812, the "IPv4 router requirements" RFC from June, 1995 has somewhat
    to say on the matter and appears to be the current applicable document.

    RFC 1878 is little more than a bunch of convenient tables.


    RFC 1812 includes the following:

    4.2.1 INTRODUCTION

    Routers MUST implement the IP protocol, as defined by [INTERNET:1].
    They MUST also implement its mandatory extensions: subnets (defined
    in [INTERNET:2]), IP broadcast (defined in [INTERNET:3]), and
    Classless Inter-Domain Routing (CIDR, defined in [INTERNET:15]).

    .... and ...

    DISCUSSION
    Previous versions of this document also noted that subnet numbers
    must be neither 0 nor -1, and must be at least two bits in length.
    In a CIDR world, the subnet number is clearly an extension of the
    network prefix and cannot be interpreted without the remainder of
    the prefix. This restriction of subnet numbers is therefore
    meaningless in view of CIDR and may be safely ignored.

  4. Re: All 0's and all 1's subnets...availables ???

    webriggs@encompasserve.org escribió:
    > In article <_Whig.2328624$kp.12532393@telenews.teleline.es>, Alberto writes:
    >> Hi. I would like more info on the following: I want to make some
    >> calculus of subnetting and I am looking for info about the numbers of
    >> subnets available.
    >> RFC 950 says I should use n bits for addressing till 2^n -2 subnets, and
    >> I should avoid all 0's and all 1's subnets.
    >> But on "VLSM" RFC 1878 it says it is not necessary to avoid these
    >> subnets. What should I do ? What is current RFC being applied ???

    >
    > RFC 1812, the "IPv4 router requirements" RFC from June, 1995 has somewhat
    > to say on the matter and appears to be the current applicable document.
    >
    > RFC 1878 is little more than a bunch of convenient tables.
    >
    >
    > RFC 1812 includes the following:
    >
    > 4.2.1 INTRODUCTION
    >
    > Routers MUST implement the IP protocol, as defined by [INTERNET:1].
    > They MUST also implement its mandatory extensions: subnets (defined
    > in [INTERNET:2]), IP broadcast (defined in [INTERNET:3]), and
    > Classless Inter-Domain Routing (CIDR, defined in [INTERNET:15]).
    >
    > ... and ...
    >
    > DISCUSSION
    > Previous versions of this document also noted that subnet numbers
    > must be neither 0 nor -1, and must be at least two bits in length.
    > In a CIDR world, the subnet number is clearly an extension of the
    > network prefix and cannot be interpreted without the remainder of
    > the prefix. This restriction of subnet numbers is therefore
    > meaningless in view of CIDR and may be safely ignored.

    well tnx for info, but what about if I dont use CIDR on my LAN ??? Would
    be possible ?
    I know 2days world IP is CIDR, but could I "mix" CIDR+classic subnetting
    for my LAN addressing, or CIDR+VLSM ???

  5. Re: All 0's and all 1's subnets...availables ???

    In article ,
    Alberto wrote:

    > well tnx for info, but what about if I dont use CIDR on my LAN ??? Would
    > be possible ?
    > I know 2days world IP is CIDR, but could I "mix" CIDR+classic subnetting
    > for my LAN addressing, or CIDR+VLSM ???


    As far as the devices are concerned, there's not really any difference.
    Classic subnetting is just a special case of CIDR.

    Pretty much any IP stack designed in the last decade should not consider
    classful addresses specially, they should do everything with addresses
    and masks.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  6. Re: All 0's and all 1's subnets...availables ???


    Alberto wrote:
    > webriggs@encompasserve.org escribió:
    > > In article <_Whig.2328624$kp.12532393@telenews.teleline.es>, Alberto writes:
    > >> Hi. I would like more info on the following: I want to make some
    > >> calculus of subnetting and I am looking for info about the numbers of
    > >> subnets available.
    > >> RFC 950 says I should use n bits for addressing till 2^n -2 subnets, and
    > >> I should avoid all 0's and all 1's subnets.
    > >> But on "VLSM" RFC 1878 it says it is not necessary to avoid these
    > >> subnets. What should I do ? What is current RFC being applied ???

    > >
    > > RFC 1812, the "IPv4 router requirements" RFC from June, 1995 has somewhat
    > > to say on the matter and appears to be the current applicable document.
    > >
    > > RFC 1878 is little more than a bunch of convenient tables.
    > >
    > >
    > > RFC 1812 includes the following:
    > >
    > > 4.2.1 INTRODUCTION
    > >
    > > Routers MUST implement the IP protocol, as defined by [INTERNET:1].
    > > They MUST also implement its mandatory extensions: subnets (defined
    > > in [INTERNET:2]), IP broadcast (defined in [INTERNET:3]), and
    > > Classless Inter-Domain Routing (CIDR, defined in [INTERNET:15]).
    > >
    > > ... and ...
    > >
    > > DISCUSSION
    > > Previous versions of this document also noted that subnet numbers
    > > must be neither 0 nor -1, and must be at least two bits in length.
    > > In a CIDR world, the subnet number is clearly an extension of the
    > > network prefix and cannot be interpreted without the remainder of
    > > the prefix. This restriction of subnet numbers is therefore
    > > meaningless in view of CIDR and may be safely ignored.

    > well tnx for info, but what about if I dont use CIDR on my LAN ??? Would
    > be possible ?


    no

    > I know 2days world IP is CIDR, but could I "mix" CIDR+classic subnetting


    yes

    classic subnetting is FLSM

    CIDR, so , apply rule from RFC 1812 , use All 0 and All 1

    > for my LAN addressing, or CIDR+VLSM ???


    yes

    similarly. use All 0 and All 1.


  7. Re: All 0's and all 1's subnets...availables ???


    Barry Margolin wrote:
    > In article ,
    > Alberto wrote:
    >
    > > well tnx for info, but what about if I dont use CIDR on my LAN ??? Would
    > > be possible ?
    > > I know 2days world IP is CIDR, but could I "mix" CIDR+classic subnetting
    > > for my LAN addressing, or CIDR+VLSM ???

    >
    > As far as the devices are concerned, there's not really any difference.
    > Classic subnetting is just a special case of CIDR.
    >
    > Pretty much any IP stack designed in the last decade should not consider
    > classful addresses specially, they should do everything with addresses
    > and masks.
    >
    > --


    given the network prefix terminology, "network prefix", "extended
    network prefix". How do we as humans refer to the "bits of subnetting",
    the difference between the "NW prefix" and "extended NW prefix". The
    term subnet , I think since VLSM, tends to refer to the whole lot, and
    is now meant to be obsoleted by 'network prefix'. So is there a term,
    besides "bits of subnetting"?

    CIDR seems to call anything after the "network prefix" the Host. which
    is unhelpful and not really the area i'm referring to. I'm referring to
    layer 2 in a "standard" 3 layer scheme that anybody always uses:
    block,subnet,host


  8. Re: All 0's and all 1's subnets...availables ???

    In article <1150326429.739656.23080@h76g2000cwa.googlegroups.c om>,
    "q_q_anonymous@yahoo.co.uk" wrote:

    > CIDR seems to call anything after the "network prefix" the Host. which
    > is unhelpful and not really the area i'm referring to. I'm referring to
    > layer 2 in a "standard" 3 layer scheme that anybody always uses:
    > block,subnet,host


    Who is this "anybody"? The advent of CIDR has made the distinction
    between networks and subnets irrelevant. All any device cares about is
    whether the address it's sending to is local or remote, i.e. whether it
    can send directly to it or must go through a router. There's no special
    significance to collecting subnets into a particular classful network.

    Originally the all-1's subnet was reserved because they had the idea
    that you might want to send a broadcast to ALL the subnets of your
    network, and you would do that using the address {net, -1, -1}. This
    turned out not to be a useful function, and few routers implemented it
    (it's difficult to avoid broadcast loops). So the only special
    significance of the network/subnet distinction was never really used,
    and CIDR eliminated this special case for good.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  9. Re: All 0's and all 1's subnets...availables ???


    Barry Margolin wrote:
    > In article <1150326429.739656.23080@h76g2000cwa.googlegroups.c om>,
    > "q_q_anonymous@yahoo.co.uk" wrote:
    >
    > > CIDR seems to call anything after the "network prefix" the Host. which
    > > is unhelpful and not really the area i'm referring to. I'm referring to
    > > layer 2 in a "standard" 3 layer scheme that anybody always uses:
    > > block,subnet,host

    >
    > Who is this "anybody"? The advent of CIDR has made the distinction
    > between networks and subnets irrelevant. All any device cares about is
    > whether the address it's sending to is local or remote, i.e. whether it
    > can send directly to it or must go through a router. There's no special
    > significance to collecting subnets into a particular classful network.
    >


    a human being might want to refer to it, since he is subnetting the
    network. He won't be
    creating subnets in the block part. There are still 3 layers there.

    what you have shown is that a computer or router wouldn't care. But I
    know that. A computer wouldn't be designing the subnetting scheme, or
    describing where he subnetted the block or even which part is the
    block.

    > Originally the all-1's subnet was reserved because they had the idea
    > that you might want to send a broadcast to ALL the subnets of your
    > network, and you would do that using the address {net, -1, -1}. This
    > turned out not to be a useful function, and few routers implemented it
    > (it's difficult to avoid broadcast loops). So the only special
    > significance of the network/subnet distinction was never really used,
    > and CIDR eliminated this special case for good.
    >


    I know, thanks


  10. Re: All 0's and all 1's subnets...availables ???

    In article <1150333213.554519.74900@y41g2000cwy.googlegroups.c om>,
    "q_q_anonymous@yahoo.co.uk" wrote:

    > Barry Margolin wrote:
    > > In article <1150326429.739656.23080@h76g2000cwa.googlegroups.c om>,
    > > "q_q_anonymous@yahoo.co.uk" wrote:
    > >
    > > > CIDR seems to call anything after the "network prefix" the Host. which
    > > > is unhelpful and not really the area i'm referring to. I'm referring to
    > > > layer 2 in a "standard" 3 layer scheme that anybody always uses:
    > > > block,subnet,host

    > >
    > > Who is this "anybody"? The advent of CIDR has made the distinction
    > > between networks and subnets irrelevant. All any device cares about is
    > > whether the address it's sending to is local or remote, i.e. whether it
    > > can send directly to it or must go through a router. There's no special
    > > significance to collecting subnets into a particular classful network.
    > >

    >
    > a human being might want to refer to it, since he is subnetting the
    > network. He won't be
    > creating subnets in the block part. There are still 3 layers there.


    Not really. You're given a block of addresses. If you have multiple
    networks at your location, you decide on an appropriate way to apportion
    the block among them, and define the network masks for each one.
    There's no need to think of it as 3 layers, you just define several
    2-layer networks that happen to span the block you were given.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  11. Re: All 0's and all 1's subnets...availables ???


    Barry Margolin wrote:
    > In article <1150333213.554519.74900@y41g2000cwy.googlegroups.c om>,
    > "q_q_anonymous@yahoo.co.uk" wrote:
    >
    > > Barry Margolin wrote:
    > > > In article <1150326429.739656.23080@h76g2000cwa.googlegroups.c om>,
    > > > "q_q_anonymous@yahoo.co.uk" wrote:
    > > >
    > > > > CIDR seems to call anything after the "network prefix" the Host. which
    > > > > is unhelpful and not really the area i'm referring to. I'm referring to
    > > > > layer 2 in a "standard" 3 layer scheme that anybody always uses:
    > > > > block,subnet,host
    > > >
    > > > Who is this "anybody"? The advent of CIDR has made the distinction
    > > > between networks and subnets irrelevant. All any device cares about is
    > > > whether the address it's sending to is local or remote, i.e. whether it
    > > > can send directly to it or must go through a router. There's no special
    > > > significance to collecting subnets into a particular classful network.
    > > >

    > >
    > > a human being might want to refer to it, since he is subnetting the
    > > network. He won't be
    > > creating subnets in the block part. There are still 3 layers there.

    >
    > Not really. You're given a block of addresses. If you have multiple
    > networks at your location, you decide on an appropriate way to apportion
    > the block among them, and define the network masks for each one.
    > There's no need to think of it as 3 layers, you just define several
    > 2-layer networks that happen to span the block you were given.
    >
    > --


    but if you were using CIDR+FLSM then in order to work out what your
    subnets are, you have to think about those "bits of subnetting" as
    distinct from the block.

    certainly, to know how many subnets there are

    furthermore, even to know if you're deling with subnet 0,1,2,3, or 4
    (relative subnet number). To know the multiple, the bit values of
    those bits of subnetting. To know the values of the interesting/split
    octet

    if you say we don't use FLSM. Surely FLSM is very convenient since it's
    presumably quicker to set up than VLSM.. It's more wasteful but it can
    still be used.


  12. Re: All 0's and all 1's subnets...availables ???

    On 2006-06-14, Barry Margolin wrote:

    > can send directly to it or must go through a router. There's no special
    > significance to collecting subnets into a particular classful network.


    There is in the network routing world, where sometimes it is highly
    desirable to collect a contiguous set of subnets or the entirety of
    a classful address range inside a routing "domain" such that the core
    of the network has the minimum of network routing information needed
    to push packets around. (This is of course a gross oversimplification of
    that for the sake of this posting.). The process in the routing world
    is called "summarization".

    > Originally the all-1's subnet was reserved because they had the idea
    > that you might want to send a broadcast to ALL the subnets of your
    > network, and you would do that using the address {net, -1, -1}. This
    > turned out not to be a useful function, and few routers implemented it


    Excellent historical note, thank you.

    /dmfh

    ----
    __| |_ __ / _| |_ ____ __
    dmfh @ / _` | ' \| _| ' \ _ / _\ \ /
    \__,_|_|_|_|_| |_||_| (_) \__/_\_\
    ----

  13. Re: All 0's and all 1's subnets...availables ???

    In article ,
    DMFH wrote:

    > On 2006-06-14, Barry Margolin wrote:
    >
    > > can send directly to it or must go through a router. There's no special
    > > significance to collecting subnets into a particular classful network.

    >
    > There is in the network routing world, where sometimes it is highly
    > desirable to collect a contiguous set of subnets or the entirety of
    > a classful address range inside a routing "domain" such that the core
    > of the network has the minimum of network routing information needed
    > to push packets around. (This is of course a gross oversimplification of
    > that for the sake of this posting.). The process in the routing world
    > is called "summarization".


    But you still don't need to think of it as a
    triple.

    Within the contiguous set of subnets you use the fine-grained network
    mask and deal with . Outside that contiguous set you use
    the summary network mask, and again deal with .

    Nothing that deals with subnets needs to know about the aggregation, and
    nothing that deals with the aggregate needs to know about the subnets,
    so there's never any need to think of it as a 3-level network.

    Of course, the guy who sets up the subnets needs to know what aggregate
    he has available to split up.

    --
    Barry Margolin, barmar@alum.mit.edu
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***

  14. Re: All 0's and all 1's subnets...availables ???

    Barry Margolin wrote:

    (snip)

    > As far as the devices are concerned, there's not really any difference.
    > Classic subnetting is just a special case of CIDR.


    Except for non-contiguous masks classically allowed, but not for CIDR.

    > Pretty much any IP stack designed in the last decade should not consider
    > classful addresses specially, they should do everything with addresses
    > and masks.


    There are still old hosts around. I have seen some that won't allow
    supernetting, because they believe the old classes. But yes, probably
    more than 10 years old.

    -- glen


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