4. SPECIFICS FOR NETWORK NAME MAPPINGS
The proposed solution uses information stored at:
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Names in the IN-ADDR.ARPA tree that correspond to host-zero IP addresses. The same method is used for subnets in a nested fashion. For example, 0.0.0.10.IN-ADDR.ARPA. for net 10.
Two types of information are stored here: PTR RRs which point to the network name in their data sections, and A RRs, which are present if the network (or subnet) is subnetted further. If a type A RR is present, then it has the address mask as its data. The general form is:
<reversed-host-zero-number>.IN-ADDR.ARPA. PTR <network-name> <reversed-host-zero-number>.IN-ADDR.ARPA. A <subnet-mask>For example:
0.0.0.10.IN-ADDR.ARPA. PTR ARPANET.ARPA.or
0.0.2.128.IN-ADDR.ARPA. PTR cmu-net.cmu.edu. A 255.255.255.0In general, this information will be added to an existing master file for some IN-ADDR.ARPA domain for each network involved. Similar RRs can be used at host-zero subnet entries.
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Names which are network names.
The data stored here is PTR RRs pointing at the host-zero entries. The general form is:
<network-name> ptr <reversed-host-zero-number>.IN-ADDR.ARPAFor example:
ARPANET.ARPA. PTR 0.0.0.10.IN-ADDR.ARPA.or
isi-net.isi.edu. PTR 0.0.9.128.IN-ADDR.ARPA.In general, this information will be inserted in the master file for the domain name of the organization; this is a different file from that which holds the information below IN-ADDR.ARPA. Similar PTR RRs can be used at subnet names.
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Names corresponding to organizations.
The data here is one or more PTR RRs pointing at the IN-ADDR.ARPA names corresponding to host-zero entries for networks.
For example:
ISI.EDU. PTR 0.0.9.128.IN-ADDR.ARPA. MCC.COM. PTR 0.167.5.192.IN-ADDR.ARPA. PTR 0.168.5.192.IN-ADDR.ARPA. PTR 0.169.5.192.IN-ADDR.ARPA. PTR 0.0.62.128.IN-ADDR.ARPA.
4.1. A simple example
The ARPANET is a Class A network without subnets. The RRs which would be added, assuming the ARPANET.ARPA was selected as a network name, would be:
ARPA. PTR 0.0.0.10.IN-ADDR.ARPA. ARPANET.ARPA. PTR 0.0.0.10.IN-ADDR.ARPA. 0.0.0.10.IN-ADDR.ARPA. PTR ARPANET.ARPA.
The first RR states that the organization named ARPA owns net 10 (It might also own more network numbers, and these would be represented with an additional RR per net.) The second states that the network name ARPANET.ARPA. maps to net 10. The last states that net 10 is named ARPANET.ARPA.
Note that all of the usual host and corresponding IN-ADDR.ARPA entries would still be required.
4.2. A complicated, subnetted example
The ISI network is 128.9, a class B number. Suppose the ISI network was organized into two levels of subnet, with the first level using an additional 8 bits of address, and the second level using 4 bits, for address masks of x'FFFFFF00' and X'FFFFFFF0'.
Then the following RRs would be entered in ISI's master file for the ISI.EDU zone:
; Define network entry isi-net.isi.edu. PTR 0.0.9.128.IN-ADDR.ARPA. ; Define first level subnets div1-subnet.isi.edu. PTR 0.1.9.128.IN-ADDR.ARPA. div2-subnet.isi.edu. PTR 0.2.9.128.IN-ADDR.ARPA. ; Define second level subnets inc-subsubnet.isi.edu. PTR 16.2.9.128.IN-ADDR.ARPA. in the 9.128.IN-ADDR.ARPA zone: ; Define network number and address mask 0.0.9.128.IN-ADDR.ARPA. PTR isi-net.isi.edu. A 255.255.255.0 ;aka X'FFFFFF00' ; Define one of the first level subnet numbers and masks 0.1.9.128.IN-ADDR.ARPA. PTR div1-subnet.isi.edu. A 255.255.255.240 ;aka X'FFFFFFF0' ; Define another first level subnet number and mask 0.2.9.128.IN-ADDR.ARPA. PTR div2-subnet.isi.edu. A 255.255.255.240 ;aka X'FFFFFFF0' ; Define second level subnet number 16.2.9.128.IN-ADDR.ARPA. PTR inc-subsubnet.isi.edu.
This assumes that the ISI network is named isi-net.isi.edu., first level subnets are named div1-subnet.isi.edu. and div2- subnet.isi.edu., and a second level subnet is called inc- subsubnet.isi.edu. (In a real system as complicated as this there would be more first and second level subnets defined, but we have shown enough to illustrate the ideas.)
4.3. Procedure for using an IP address to get network name
Depending on whether the IP address is class A, B, or C, mask off the high one, two, or three bytes, respectively. Reverse the octets, suffix IN-ADDR.ARPA, and do a PTR query.
For example, suppose the IP address is 10.0.0.51.
1. Since this is a class A address, use a mask x'FF000000' and
get 10.0.0.0.
2. Construct the name 0.0.0.10.IN-ADDR.ARPA.
3. Do a PTR query. Get back
0.0.0.10.IN-ADDR.ARPA. PTR ARPANET.ARPA.
4. Conclude that the network name is "ARPANET.ARPA."
Suppose that the IP address is 128.9.2.17.
1. Since this is a class B address, use a mask of x'FFFF0000'
and get 128.9.0.0.
2. Construct the name 0.0.9.128.IN-ADDR.ARPA.
3. Do a PTR query. Get back
0.0.9.128.IN-ADDR.ARPA. PTR isi-net.isi.edu
4. Conclude that the network name is "isi-net.isi.edu."
4.4. Procedure for finding all subnets involved with an IP address
This is a simple extension of the IP address to network name method. When the network entry is located, do a lookup for a possible A RR. If the A RR is found, look up the next level of subnet using the original IP address and the mask in the A RR. Repeat this procedure until no A RR is found.
For example, repeating the use of 128.9.2.17.
1. As before construct a query for 0.0.9.128.IN-ADDR.ARPA.
Retrieve:
0.0.9.128.IN-ADDR.ARPA. PTR isi-net.isi.edu.
A 255.255.255.0
2. Since an A RR was found, repeat using mask from RR
(255.255.255.0), constructing a query for
0.2.9.128.IN-ADDR.ARPA. Retrieve:
0.2.9.128.IN-ADDR.ARPA. PTR div2-subnet.isi.edu.
A 255.255.255.240
3. Since another A RR was found, repeat using mask
255.255.255.240 (x'FFFFFFF0'). constructing a query for
16.2.9.128.IN-ADDR.ARPA. Retrieve:
16.2.9.128.IN-ADDR.ARPA. PTR inc-subsubnet.isi.edu.
4. Since no A RR is present at 16.2.9.128.IN-ADDR.ARPA., there
are no more subnet levels.