IPs , Netmasks and Broadcast.

John Oxley john at yoafrica.com
Thu Sep 15 04:56:21 PDT 2005


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


More information about the freebsd-questions mailing list