firstly, like someone else said, its for a bit of structure, so that the necessary people can see in what region the IP is in.
secondly, yes normally unused IPs have dns entries, especially for dial-up users and DSL users. an isp i worked at previously have modem1.ispname.co.za to modem99.ispname.co.za, so that when users dialled up (seeing as DSL "dials" to some extent), they would have a hostname. just because an IP resolves to a hostname, doesn't mean its in use, heck, these days with security being such an issue, pinging a host and not getting a reply doesn't mean the host is down, just means ICMP replies aren't finding their way back to you. a lot of larger companies (from what i've seen/experienced in .za anyway) & universities use DHCP for workstations, and if they were handing out public IPs, chances are they'ld have some sort of DNSing in place, so might as well allocate IP's z.z.z.a-z.z.z.b for example once off. in the case of a larger company/varsity, i'm sure they too would have seperate scopes for each dept. for admin purposes. On Thu, 31 Jul 2003, Terry wrote: > Hi, > > I've googled but haven't really come across anything that answers my > questions. Is it common practice for ISPs to allocate a block of > addresses to a customer and put in DNS records for ones that are > unused? > > For example, xxx.8-xxx.15 is assigned to the customer. Customer uses > xxx.9 for the router and xxx.10 for web server. xxx.11-14 are unused. > I would expect to see DNS records for xxx.9 and xxx.10 but not for the > rest of the block. Am I wrong? If there are DNS records for the rest > of the IPs, why is that? Is there some security reason for doing > this? > > -- > Regards, > Terry > > > --------------------------------------------------------------------------- > ---------------------------------------------------------------------------- > --------------------------------------------------------------------------- ----------------------------------------------------------------------------
