Re: tlug: Netmask question

[Julien Beasley <jbb@example.com>]

On Mon, 21 Feb 2000, Selva Nair wrote:

> 
> 
> On Mon, 21 Feb 2000, Julien Beasley wrote:
> 
> >  Hello everyone,
> >  
> >  I have a problem that I'm sure is simple to solve, but I've been stumped
> >  on this. 
> >  
> >  When I connect via ppp to my ISP via gol, I am assigned an ip that is one
> >  of the following forms:
> >  
> >  xxx.yyy.8.zzz
> >  xxx.yyy.9.zzz
> >  xxx.yyy.10.zzz
> 
> Guess you dont mean three identical digits by xxx, yyy etc. It may be
> useful to know the last byte to see (from the subnet mask) which subnet
> you belong to in each case. 

No, they are not the same three digits =). I just didn't remember off the
top of my head what they were.

> >  When I connect, I can always ping outside computers and do nslookup
> >  regardless of which kind of ip I am assigned. However, I can only telnet,
> >  irc, and www if I am assiged an ip of the form:
> >  
> >  xxx.yyy.8.zzz
> >  
> >  
> >  So I can ping regardless of which ip I was assigned, but I can only use
> >  other protocols if I have an ip that ends in xxx.yyy.8.zzz
> >  
> >  I happen to be using IP masquerading on this computer but I don't _think_
> >  that has anything to do with it. The other computers behind the masq
> >  behave the same way as the gateway computer. If the linux box has an 8.zzz
> >  ip, the computer behind the masq can www and telnet. If not, it can only
> >  ping.
> >  
> >  If I do an /sbin/ifconfig, it tells me that the netmask on my ppp
> >  connection is 225.225.225.225. Is that wrong? 
> 
> If that is really the (sub)-netmask it is wrong. But do you mean
> 255.255.255.225? Even that is unusual as 225 will be a non-contigous bit
> mask, something not very common (though the standard (RFC 950) does
> permit it). I suppose that you are using dhcp, so that the netmask and
> gateway addresses are obtained from the ISP's server. Do you have the 
> same subnet mask for the all three IP addresses you mentioned? Also can
> we see the output of ipconfig ?

Sorry, typ. it's 255.255.255.255.
Yes, the subnet mask is always the same. Tomorrow I'll bring my laptop to
work and cat ifconfig and netstat

> >  How can it be that I can ping in any case but I can only telnet and www if
> >  I have an 8.zzz ip?
> >  
> 
> I have no clear idea, but a possible guess is that you have some ppp
> option (say header compression) which your server doesn't support.
> ICMP packets of ping may get thru but not TCP packets of telnet.
> But first let us get the netmask right.

Ok, thank you. I'll have more detail tomorrow

Julien

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