IPs , Netmasks and Broadcast.

Jerry McAllister jerrymc at clunix.cl.msu.edu
Thu Sep 15 07:30:38 PDT 2005


> 
> Jerry,
> 
> I see, you changed the real netmask in the second and third group to 255.

No, I changed the alias' netmask, not the "real" one.

> Hmm, I went through this before, and unless I added 1 ip from each block 
> with the real netmask, the ip wouldnt take.
> 
> FYI when my ISP sends me a new block, they always give me the network, 
> router, broadcas address etc and I run them through ip calc, and they are 
> always correct.
> 
> Either way, I better revisit the Handbook/Manpage as it seems that I am 
> asking the same question(s) that I thought were answered 5 years ago...

that is always a good idea.

////jerry

> 
> Thanks for the help Jerry,
> 
> -Grant
> 
> ----- Original Message ----- 
> From: "Jerry McAllister" <jerrymc at clunix.cl.msu.edu>
> To: <gpeel at thenetnow.com>
> Cc: <freebsd-questions at freebsd.org>; "John Oxley" <john at yoafrica.com>
> Sent: Thursday, September 15, 2005 9:31 AM
> Subject: Re: IPs , Netmasks and Broadcast.
> 
> 
> >>
> >> Jerry,
> >>
> >> I apologise. Its late in the day for me and the email addresses are 
> >> blending
> >> into one :-)
> >>
> >> Anyways, so the examples I put together are correct. Good. So I wonder 
> >> why,
> >> the 1 IP I have, won't work with the real netmask, but only with the 255.
> >> One more item of note, I see in dmesg that when it tries to apply it, the
> >> message:
> >
> > Look carefully.   I changed one thing in each group.
> >
> > Also, if you have a block of 128, your main mask would
> > probably not be 255.255.2555.0.  I think it would be 255.255.255.128.
> >
> > Anyway, all the aliases for any one NIC card must be 255.255.255.255.
> >
> >>
> >> arplookup xxx.xxx.xxx.xxx failed: host is not on local network
> >
> > Probably because the ifconfig didn't work right with the wrong
> > netmasks.
> >
> >
> >> -Grant
> >>
> >> P.S. This is a small block, only 30 ips. The other 29, including 1 with 
> >> the
> >> real netmask, is on another machine working fine, right beside the 
> >> machine I
> >> am referencing.
> >
> > You will have to calculate the mask somewhat based on the switch/router
> > segment you are on.    It is too early for me to think it out.
> >
> > ////jerry
> >
> >>
> >>
> >> ----- Original Message ----- 
> >> From: "Jerry McAllister" <jerrymc at clunix.cl.msu.edu>
> >> To: <gpeel at thenetnow.com>
> >> Cc: "John Oxley" <john at yoafrica.com>; <freebsd-questions at freebsd.org>
> >> Sent: Thursday, September 15, 2005 8:56 AM
> >> Subject: Re: IPs , Netmasks and Broadcast.
> >>
> >>
> >> >>
> >> >> Thanks John,
> >> >>
> >> >> What you are saying make perfect sense based on what I've had to do in
> >> >> the
> >> >> past. Just to clarify ...
> >> >>
> >> >> Lets say I have 5 machines (which I do)
> >> >>
> >> >> And lets say I have 10 blocks of IP address from my upline (which I 
> >> >> do)
> >> >>
> >> >> 1.1.1.1/24
> >> >> 2.2.2.2/24
> >> >> 3.3.3.3/24
> >> >> 4.4.4.4/24
> >> >> ...
> >> >>
> >> >> And, lets say that I need to set up various IPs, from each of the 
> >> >> groups
> >> >> above, on all 5 of the servers.
> >> >>
> >> >> To keep the explanation easy to follow, I will put what I might have 
> >> >> in
> >> >> rc.conf for each machine, Leaving out the gateway IPs as they are not
> >> >> realevent to the question...my router setting work fine.
> >> >
> >> > No, this is not what I said.   I don't know if anyone else has posted
> >> > on this since then.   But,
> >> >
> >> > The next two chunks below represent what I said.
> >> >
> >> > ////jerry
> >> >
> >> > Sever 1:
> >> >
> >> > hostname="machine1.mydomain.com"
> >> > defaultrouter="some ip here"
> >> > ifconfig_em0="inet 1.1.1.2 netmask 255.255.255.0"
> >> > ifconfig_em0_alias0="inet 1.1.1.3 netmask 255.255.255.255"
> >> > ifconfig_em0_alias1="inet 1.1.1.4 netmask 255.255.255.255"
> >> > ...
> >> > ifconfig_em0_alias2="inet 2.2.2.3 netmask 255.255.255.255"
> >> > ifconfig_em0_alias3="inet 2.2.2.4 netmask 255.255.255.255"
> >> > ifconfig_em0_alias4="inet 2.2.2.5 netmask 255.255.255.255"
> >> > ...
> >> > ifconfig_em0_alias5="inet 3.3.3.4 netmask 255.255.255.255"
> >> > ifconfig_em0_alias6="inet 3.3.3.5 netmask 255.255.255.255"
> >> > ifconfig_em0_alias7="inet 3.3.3.6 netmask 255.255.255.255"
> >> > ...
> >> >
> >> > Sever 2:
> >> >
> >> > hostname="machine2.mydomain.com"
> >> > defaultrouter="some ip here"
> >> > ifconfig_em0="inet 1.1.1.5 netmask 255.255.255.0"
> >> > ifconfig_em0_alias0="inet 1.1.1.6 netmask 255.255.255.255"
> >> > ifconfig_em0_alias1="inet 1.1.1.7 netmask 255.255.255.255"
> >> > ...
> >> > ifconfig_em0_alias2="inet 2.2.2.6 netmask 255.255.255.255"
> >> > ifconfig_em0_alias3="inet 2.2.2.7 netmask 255.255.255.255"
> >> > ifconfig_em0_alias4="inet 2.2.2.8 netmask 255.255.255.255"
> >> > ...
> >> > ifconfig_em0_alias5="inet 3.3.3.7 netmask 255.255.255.255"
> >> > ifconfig_em0_alias6="inet 3.3.3.8 netmask 255.255.255.255"
> >> > ifconfig_em0_alias7="inet 3.3.3.9 netmask 255.255.255.255"
> >> > ...
> >> >
> >> >
> >> >  -----------------------------------------------------------------
> >> >
> >> > Sever 1:
> >> >>
> >> >> hostname="machine1.mydomain.com"
> >> >> defaultrouter="some ip here"
> >> >> ifconfig_em0="inet 1.1.1.2 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias0="inet 1.1.1.3 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias1="inet 1.1.1.4 netmask 255.255.255.255"
> >> >> ...
> >> >> ifconfig_em0_alias2="inet 2.2.2.3 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias3="inet 2.2.2.4 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias4="inet 2.2.2.5 netmask 255.255.255.255"
> >> >> ...
> >> >> ifconfig_em0_alias5="inet 3.3.3.4 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias6="inet 3.3.3.5 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias7="inet 3.3.3.6 netmask 255.255.255.255"
> >> >> ...
> >> >>
> >> >>
> >> >> Sever 2:
> >> >>
> >> >> hostname="machine2.mydomain.com"
> >> >> defaultrouter="some ip here"
> >> >> ifconfig_em0="inet 1.1.1.5 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias0="inet 1.1.1.6 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias1="inet 1.1.1.7 netmask 255.255.255.255"
> >> >> ...
> >> >> ifconfig_em0_alias2="inet 2.2.2.6 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias3="inet 2.2.2.7 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias4="inet 2.2.2.8 netmask 255.255.255.255"
> >> >> ...
> >> >> ifconfig_em0_alias5="inet 3.3.3.7 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias6="inet 3.3.3.8 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias7="inet 3.3.3.9 netmask 255.255.255.255"
> >> >> ...
> >> >>
> >> >> Sever 2:
> >> >>
> >> >> hostname="machine3.mydomain.com"
> >> >> defaultrouter="some ip here"
> >> >> ifconfig_em0="inet 1.1.1.8 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias0="inet 1.1.1.9 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias1="inet 1.1.1.10 netmask 255.255.255.255"
> >> >> ...
> >> >> ifconfig_em0_alias2="inet 2.2.2.9 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias3="inet 2.2.2.10 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias4="inet 2.2.2.11 netmask 255.255.255.255"
> >> >> ...
> >> >> ifconfig_em0_alias5="inet 3.3.3.10 netmask 255.255.255.0"
> >> >> ifconfig_em0_alias6="inet 3.3.3.11 netmask 255.255.255.255"
> >> >> ifconfig_em0_alias7="inet 3.3.3.12 netmask 255.255.255.255"
> >> >> ...
> >> >>
> >> >> Server 3, 4 and 5 similar with repeating patterns.
> >> >>
> >> >> So the original question was: When setting up an IP from any given 
> >> >> block,
> >> >> do, or should I, need to use the real netmask that my ISP has assinged 
> >> >> me
> >> >> (for the first), then, the single host (255) netmask for each of the 
> >> >> rest
> >> >> (Again, from a specific IP block?
> >> >>
> >> >> I hope it is because I have hundreds of IPs set up that way.
> >> >>
> >> >> The reason I ask this question (again) is because I have one IP that 
> >> >> does
> >> >> not want to work using the above paradigm. That is to say, that it IS 
> >> >> the
> >> >> only IP from a specific block, on a particular machine, and it won't 
> >> >> take
> >> >> using the real netmask, it will only work using the 255 netmask.
> >> >>
> >> >> Sincerly,
> >> >>
> >> >> -Confused
> >> >> (Grant).
> >> >>
> >> >>
> >> >>
> >> >>
> >> >>
> >> >>
> >> >>
> >> >> ----- Original Message ----- 
> >> >> From: "John Oxley" <john at yoafrica.com>
> >> >> To: "Grant Peel" <gpeel at thenetnow.com>
> >> >> Cc: <freebsd-questions at freebsd.org>
> >> >> Sent: Thursday, September 15, 2005 7:55 AM
> >> >> Subject: Re: IPs , Netmasks and Broadcast.
> >> >>
> >> >>
> >> >> > On Wed, Sep 14, 2005 at 12:00:12PM -0400, Grant Peel wrote:
> >> >> >> Now I am really confused.
> >> >> > First of all install ipcalc NOW.
> >> >> >
> >> >> >> Up until this morning I thought I had a good handle on when I need 
> >> >> >> to
> >> >> >> use
> >> >> >> the real Netmask and Broadcast.
> >> >> >>
> >> >> >> For example, If I have 3 servers, and my upline asigns me a block 
> >> >> >> of
> >> >> >> 128
> >> >> >> addresses, for the first ip used (from this block) on each server, 
> >> >> >> I
> >> >> >> would
> >> >> >> need to specify the IP, true netmask and a broadcase. Then, when 
> >> >> >> using
> >> >> >> more
> >> >> >> IPs from that block, I would use a 255.255.255.255 netmask and a
> >> >> >> broadcast
> >> >> >> equal to the IP.
> >> >> >
> >> >> > No, a 255.255.255.255 netmask specifies a single host. If your ISP 
> >> >> > has
> >> >> > given you 128 IPs, that is a /25 or netmask 255.255.255.128 as shown 
> >> >> > :
> >> >> > sysjo at hades:~$ ipcalc 192.168.254.0/25
> >> >> > Address:   192.168.254.0        11000000.10101000.11111110.0 0000000
> >> >> > Netmask:   255.255.255.128 = 25 11111111.11111111.11111111.1 0000000
> >> >> > Wildcard:  0.0.0.127            00000000.00000000.00000000.0 1111111
> >> >> > =>
> >> >> > Network:   192.168.254.0/25     11000000.10101000.11111110.0 0000000
> >> >> > HostMin:   192.168.254.1        11000000.10101000.11111110.0 0000001
> >> >> > HostMax:   192.168.254.126      11000000.10101000.11111110.0 1111110
> >> >> > Broadcast: 192.168.254.127      11000000.10101000.11111110.0 1111111
> >> >> > Hosts/Net: 126                   Class C, Private Internet
> >> >> >
> >> >> > So all your machines would have the same network and broadcast, and 
> >> >> > you
> >> >> > would make them all use your gateway box as the default route.
> >> >> >
> >> >> > As you can see, a 255.255.255.255 netmask (/32) is only one host:
> >> >> > sysjo at hades:~$ ipcalc 192.168.254.34/32
> >> >> > Address:   192.168.254.34       11000000.10101000.11111110.00100010
> >> >> > Netmask:   255.255.255.255 = 32 11111111.11111111.11111111.11111111
> >> >> > Wildcard:  0.0.0.0              00000000.00000000.00000000.00000000
> >> >> > =>
> >> >> > Hostroute: 192.168.254.34       11000000.10101000.11111110.00100010
> >> >> > Hosts/Net: 1                     Class C, Private Internet
> >> >> >
> >> >> >
> >> >> > What you need to do is use IP calc to see what the network and
> >> >> > broadcast
> >> >> > are and set those, although on FreeBSD 4.X (I think anyway, someone
> >> >> > correct me if I am wrong) you can just put in your /etc/rc.conf
> >> >> > ifconfig_bge0="inet 192.168.254.32/25"
> >> >> > or
> >> >> > ifconfig_bge0="inet 192.168.254.32 netmask 255.255.255.128"
> >> >> >
> >> >> > And FreeBSD will work out the network and broadcast for you.
> >> >> >
> >> >> >> Today, I was in this exact position where I was tring to add an 
> >> >> >> (the
> >> >> >> first
> >> >> >> one one THAT machine, from that block) IP from a block that is 
> >> >> >> almost
> >> >> >> completely used up on another server, and the one I was adding it 
> >> >> >> to
> >> >> >> would
> >> >> >> not take it. When I tried adding it with a 255.255.255.255 netmask,
> >> >> >> and a
> >> >> >> broadcast eaqual to the amount of IPs from that block - it worked.
> >> >> >
> >> >> > Like I said before, there is only 1 IP in a /32 block, and the
> >> >> > broadcast
> >> >> > will be either a.b.c.127 or a.b.c.255 depending whether you have the
> >> >> > first or second subnet inside that class C.
> >> >> >
> >> >> > Hope this helps
> >> >> >
> >> >> > -John
> >> >> >
> >> >> >
> >> >>
> >> >>
> >> >> _______________________________________________
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> >> >>
> >> >
> >> >
> >> >
> >>
> >>
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> >>
> >
> >
> > 
> 
> 
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