IPs , Netmasks and Broadcast.

Jerry McAllister jerrymc at clunix.cl.msu.edu
Thu Sep 15 05:56:39 PDT 2005


> 
> 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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