OIT Network Systems

OIT Static IP Recycling Facility (SIRF)

The OIT Static IP Recycling Facility (SIRF) locates Static IP Addresses that are assigned in the Princeton University Host Database, but have not been used for a long time. It recycles those IP addresses, so they may be re-assigned for the use of other devices. This helps the University preserve a supply of IP addresses.

Contents

  1. SIRF in a Nutshell
  2. What's the Purpose of SIRF?
  3. Which IP Addresses are Subject to SIRF?
  4. How Does SIRF Decide to Recycle an IP Address?
  5. What Happens When SIRF Recyles an IP Address?
  6. What Should I Do If My IP Address Is Being Recycled?
  7. Appendix: Why is There a recyclenet?
  8. Appendix: Why is there no online way to "renew" an unused OIT Static IP Address?
  9. If You Need Additional Assistance
  10. Related Resources

SIRF in a Nutshell

SIRF monitors most OIT Static IP Addresses. When it detects that one of these IP addresses has not been used for a year, SIRF recycles the IP address. This allows OIT to re-assign the IP address to another device in the future.

A week before an IP address is due to be recycled, SIRF sends email to the Technical Contacts listed in the device's Host Database entry. This email reminds the customer that the IP address has not been used for a long time. It explains that the IP address may soon be recycled, if the address is not used before a specified date (the recycling day) in the near future.

If the IP address is used before recycling day, SIRF does not recycle the IP address. No further email about the IP address being recycled is sent. (The same is true if the IP address is deleted from the Host Database.)

Otherwise, SIRF recycles the IP address when recycling day arrives. It sends email to the Technical Contacts listed in the device's Host Database entry, notifying them that the IP address will be recycled. SIRF changes the device's Host Database entry to remove the OIT Static IP address, replacing it with a new IP address on recyclenet. Another piece of email showing the outcome of the change is sent to the Technical Contacts.

In most cases, there's nothing you need to do when your device's IP address is recycled:

In most cases, that's all you'll probably need to know about SIRF. If you need more detailed information, read on.


What's the Purpose of SIRF?

There is a limited supply of globally routable portable IPv4 addresses. While the University is fortunate to have a large number of these addresses allocated for its use, the number is still finite. If not actively managed, shortages can become an issue.

It is essentially impossible for us to obtain additional globally routable portable IPv4 addresses. Any additional IPv4 address available today are either non-portable (they must be changed when the University changes Internet Service Providers) or are not globally-routable (they cannot be used to communicate outside the campus network).

Additionally, several subnets of the campus network "run out" of IP addresses each year. When that happens, we must renumber the IP addresses of many customer devices to distribute them on the University's remaining unused address space. This process is inconvenient for our customers. While some renumbering will still be necessary, SIRF helps conserve IP addresses so that we avoid unnecessary renumbering.

When a device is registered in the Princeton University Host Database, an OIT Static IP Address may be assigned to each of the device's network interfaces. At that time, the IP address is reserved solely for the use of that device.

If the device was registered in the Host Database as a student's Dormnet subscription, OIT automatically deletes the entry from the Host Database after the student leaves the University, or the student no longer eligible for a Dormnet subscription. (Specifically, the Host Database entry is deleted when the person's OIT netid goes away, or when the person is no longer a currently-enrolled undergraduate or graduate student, whichever comes first.) When the entry is deleted from the Host Database (or at any time the entry is unsubscribed from Dormnet), all IP addresses that were in the Host Database entry (including any that were OIT Static IP Addresses) are reclaimed so OIT may assign them to other devices as needed.

If the device was registered in the Host Database as an "office" device (i.e. not as part of a Dormnet subscription), it is the responsibility of someone responsible for the device to delete the entry from the Host Database once that device will no longer be used on the campus network. If the Host Database entry contained any OIT Static IP Addresses, deleting the entry makes those addresses available so OIT may assign them to other devices as needed.

However, sometimes an individual responsible for an office device forgets to remove its entry from the Host Database after the device has been retired. When this happens, any OIT Static IP Addresses in that Host Database entry remain unavailable for re-assignment. This wastes IP addresses.

SIRF is intended to reduce this problem by monitoring the OIT Static IP Addresses assigned to "office" entries in the Host Database. When SIRF detects that one of these IP addresses has not been used on the campus network for a year or more, it "recycles" the IP address and notifies the people responsible for that Host Database entry.


Which IP Addresses are Subject to SIRF?

SIRF recycles certain kinds of IP addresses; not all IP addresses in the Host Database are subject to SIRF.


How Does SIRF Decide to Recycle an IP Address?

On most OIT networks, a daily record is made of all the OIT IP addreses that were in-use (even briefly) on that day.

By "in use," we mean that some device said "I'm using this IP address" at some point during the day while it was attached to the subnet appropriate for that IP address.

(For the technically inclined, what happens is this: We retrieve a copy of the IP ARP cache from all OIT IP routers periodically throughout the day. We do so at a rate high enough to collect entries before they would time out from the router's ARP cache. This provides a record of all IP addresses that appeared in response to IP ARP Requests sent by the router during the day.)

Periodically, SIRF reviews all the IP addresses assigned in the Princeton University Host Database. For those IP addresses that are subject to SIRF, it checks to see the last day the IP address was used. If the IP address has not been used for a year or more, SIRF decides to recycle the IP address.

Keep in mind that this doesn't mean SIRF believes the device has been unused. It only means that the particular OIT Static IP Address has been unused in a long time. The device may be in active use on the campus network -- just not using that OIT Static IP Address. Perhaps the device is attached via a different network interface. Or perhaps it's attached to a different subnet, using OIT Mobile IP Service. Or perhaps the device is using OIT Wireless Service, which never uses any OIT Static IP Addresses.


What Happens When SIRF Recyles an IP Address?

A week before an IP address is due to be recycled, SIRF sends email to the Technical Contacts listed in the device's Host Database entry. This email reminds the customer that the IP address has not been used for a long time; it shows the last day the IP address was used on the campus network. This mail explains that the IP address may soon be recycled, if the address is not used before a specified date (the recycling day) in the near future. Here is a sample of the reminder mail.

If the IP address is used before the recycling day that appears in the mail, SIRF will not recycle the IP address. In that situation, no further email about the IP address being recycled is sent.

If someone responsible for the Host Database entry deletes the entry from the Host Database before recycling day, or changes the Host Database entry so that IP address is removed from the Host Database entry, SIRF will not recycle the IP address. In that situation, no further email about the IP address being recycled is sent.

Otherwise, when recycling day arrives, SIRF recycles the IP address. It sends email to the Technical Contacts listed in the device's Host Database entry, notifying them that the IP address will be recycled. Here is a sample of that notification mail.

Within three business days of sending that notification email, SIRF actually changes the device's Host Database entry to remove the IP address, replacing it with a new IP address on recyclenet. When SIRF changes the entry, another message is emailed to the Technical Contacts listed in the entry, showing the outcome of the change.


What Should I Do If My IP Address Is Being Recycled?

In most cases, there's nothing you need to do.

If Your Device Will Not Be Reconnected

This is the most common situation.

You need take no action. But it's a good idea to delete the entry from the Host Database, as some entries result in charges.

If the Network Interface is Connected via OIT Wireless Service

If the device's network interface has been relying solely on OIT Wireless Service for the past year, then it hasn't used its OIT Static IP Address during that time.

You need take no action.

There is no need for your device's Host Database entry to retain the old OIT Static IP Address; your device hasn't been using that IP address.

When the old IP address is recycled, OIT changes the device's Host Database entry so the network interface is homed on recyclenet. (The interface is assigned a new IP address on the recyclenet subnet. This new IP address is not an OIT Static IP Address; it acts as a placeholder in the Host Database entry.)

After the old IP address has been recycled, your network interface will still be able to use the same services it has been using (OIT Wireless Service and/or OIT Mobile IP Service, for example) with no change on your part.

If the Network Interface is Connected and Relying on OIT Mobile IP Service

If the device's network interface has been relying solely on OIT Mobile IP Service for the past year, then it hasn't used its OIT Static IP Address during that time.

This can happen if the device is registered with an OIT Static IP Address for use with OIT Ethernet Service, but has been visiting campus networks other than the one on which its OIT Static IP Address is allocated.

It can also happen if the device has been using solely OIT Mobile IP Service, but in such a way that it has not been declared an OIT Mobile IP Camper.

In either case, you need take no action.

There is no need for your device's Host Database entry to retain the old OIT Static IP Address; your device hasn't been using that IP address.

When the old IP address is recycled, OIT changes the device's Host Database entry so the network interface is homed on recyclenet. (The interface is assigned a new IP address on the recyclenet subnet. This new IP address is not an OIT Static IP Address; it acts as a placeholder in the Host Database entry.)

After the old IP address has been recycled, your network interface will still be able to use the same services it has been using (OIT Mobile IP Service and/or OIT Wireless Service, for example) with no change on your part.

If at some time in the future, your device's network interface uses OIT Mobile IP Service in such a way that it is declared an OIT Mobile IP Camper, OIT would automatically change the device's Host Database entry to assign that network interface a new OIT Static IP Address appropriate for the network on which it is camping. Email would be sent to the Technical Contacts that appear in the Host Database entry.

If the Network Interface Will Not Be Reconnected Until After the IP Address is Recycled, and it Uses DHCP

In this situation, there is usually no need for your device to retain the same IP address. Since your device uses DHCP (the Dynamic Host Configuration Protocol) to learn the IP address it should use, your device would be able to function if DHCP were to provide it with a different IP address. As long as no other devices on the network care about the specific IP address your device uses, you'll likely have no difficulty.

In this situation, you usually need take no action.

When the old IP address is recycled, OIT changes the device's Host Database entry so the network interface is homed on recyclenet. (The interface is assigned a new IP address on the recyclenet subnet. This new IP address is not an OIT Static IP Address; it acts as a placeholder in the Host Database entry.)

If reconnected via OIT Ethernet Service:

After the old IP address has been recycled, if you reconnect the network interface via OIT Ethernet Service and it uses DHCP to learn the IP address it should use, it will be loaned an OIT Mobile IP Service Address. (This assumes the device is eligible for OIT Mobile IP Service, as are most workstations, and is attached to an OIT Ethernet network where OIT Mobile IP Service is available, which includes most campus networks.)

The network interface uses OIT Mobile IP Service because it is attached to an OIT network other than its home network. (Its home network is currently recyclenet; an imaginary network not wired to any location on campus. Whatever network your devices has been attached to, you can be sure it's something other than recyclenet.)

If the network interface continues to use OIT Mobile IP Service on the same network for a period of time (currently 14 days), OIT identifies the device as a Mobile IP Camper. At that time, OIT automatically changes the device's Host Database entry to assign that network interface a new OIT Static IP Address on that network. Email is sent to the Technical Contacts that appear in the Host Database entry. If you'd like more information about how this works, see OIT Mobile IP Service: Mobile IP Campers. (Of course, you are also welcome to change the Host Database entry yourself to reflect the device's new home network, prior to OIT doing this automatically.)

In the uncommon case that the network interface uses DHCP but is not eligible for OIT Mobile IP Service or is attached to a network where OIT Mobile IP Service is not available, then when you re-attach it to the campus network, it will not obtain an OIT Mobile IP Service Address. At that time, you will need to update the device's Host Database entry; change the network interface in the entry to specify the IP subnet to which you have re-attached that network interface. The Princeton Network ("subnet") List contains a list of the IP subnets in each campus building. After the change is processed, your device's network interface will be able to use DHCP to learn its new OIT Static IP Address.

If reconnected via OIT Wirelesss Service:

After the old IP address has been recycled, if you reconnect the network interface via OIT Wireleess Service and it uses DHCP to learn the IP address it should use, it will be loaned an OIT Wireless Service NAT Inside IP Address.

OIT Wireless Service never uses any OIT Static IP Addresses.

If the Network Interface Will Not Be Reconnected Until After the IP Address is Recycled, and it Uses BootP

Some network interfaces do not use DHCP to learn the IP address they should use, but instead use an older protocol called "BootP" (the "Boot Protocol") to learn this information.

When the old IP address is recycled, OIT changes the device's Host Database entry so the network interface is homed on recyclenet. (The interface is assigned a new IP address on the recyclenet subnet. This new IP address is not an OIT Static IP Address; it acts as a placeholder in the Host Database entry.)

After the old IP address has been recycled, when you reconnect the network interface to an OIT network and it uses BootP to learn the IP address it should use, it will not be able to obtain an IP address.

This is because BootP will only provide an IP address to a device attached to its home network (the IP subnet that appears in the Host Database entry). Your network interface's home network is now recyclenet (an imaginary network), while the network to which it is attached is some other network. As a result, OIT BootP Service will not provide an IP address to that network interface.

If your network interface is able to use DHCP, you might choose to reconfigure it to use DHCP to learn its IP address, instead of using BootP.

Doing so will allow it to obtain an OIT Mobile IP Service Address when your device is eligible for OIT Mobile IP Service, and is attached via OIT Ethernet Service to a campus network where OIT Mobile IP Service is available. And doing so will also allow your device to obtain an OIT Wireless Service NAT Inside IP Address if it is attached via OIT Wireless Service.

If you do this, this puts your device into the situation described above, in the section entitled If Your Device Will Not Be Reconnected Until After the IP Address is Recycled, and the Device Uses DHCP.

If you must continue to use BootP instead of DHCP, then to resume using your network interface on the campus network via OIT Ethernet Service you will need to update its entry in the Host Database. Change the network interface in the entry to specify the IP subnet to which you have attached that network interface. The Princeton Network ("subnet") List contains a list of the IP subnets in each campus building. After the change is processed, your device's network interface will be able to use BootP to learn its new OIT Static IP Address while attached via OIT Ethernet Service to its "home" network.

If the Network Interface Will Not Be Reconnected Until After the IP Address is Recycled, and it is Manually Configured with an IP Address

When the old IP address is recycled, OIT changes the device's Host Database entry so the network interface is homed on recyclenet. (The interface is assigned a new IP address on the recyclenet subnet. This new IP address is not an OIT Static IP Address; it acts as a placeholder in the Host Database entry.)

It is your responsibility to reconfigure your device's network interface so that it is no longer configured to use the old IP address. That IP address is no longer "yours"; OIT may re-assign it to another device. If you were to reconnect your device's network interface to the campus network in the future while it was was still configured to use the old IP address, it would eventually interfere with service to another device.

When you wish to reconnect your device's network interface to the campus network, you will need to update its entry in the Host Database. Change the network interface in the entry to specify the IP subnet to which you have attached that network interface. The Princeton Network ("subnet") List contains a list of the IP subnets in each campus building. When the change is processed, mail is sent to the Technical Contacts that appear in the Host Database entry. That mail shows the new OIT Static IP Address assigned to the network interface. Reconfigure the device's network interface it so it uses the newly assigned OIT Static IP Address.

If You Do Not Want the IP Address to be Recycled

If you do not want the IP address to be recycled, you will need to take action.

Before recycling day, reconnect your device to the campus network in such a way that it uses the IP address.

You will need to attach the device's network interface that is assigned the IP address in question. For example, if the device has both an Ethernet interface and a Wireless interface, and SIRF has indicated it will recycle the IP address assigned to the Ethernet interface, be sure to attach the Ethernet interface to the network.

Attach the network interface to its home network -- the IP subnet that appears in the Host Database entry for that network interface. For example, if the IP address in question is on the musenet IP subnet and belongs to the device's Ethernet interface, then be sure to attach the device's Ethernet interface to musenet. (If you were to attach the interface to another subnet, your device would not really be able to use the IP address due to be recycled.) The Princeton Network ("subnet") List contains a list of the IP subnets in each campus building.

While attached in this way, make use of the device so it uses IP; it only need do so momentarily, so we record the IP address was "in use."

Once you do so, no further mail about the IP address being recycled is sent; since the IP address has been used recently, SIRF no longer will decide to recycle it. You do not need to contact OIT.


Appendix: Why is There a recyclenet?

This section provides some advanced information about the use of the recyclenet subnet. You can skip this material unless you're curious about the details.

When we recycle an OIT Static IP Address, we replace it with a new IP address selected from the recyclenet IP subnet.

The recyclenet subnet is an "imaginary" one; no location on campus is actually wired to this network. A recyclenet IP addresses would never be used by a device on the campus network.

The recyclenet IP addresses are not as valuable as the IP addresses we recycle, because the recyclenet IP addresses are not globally routable portable IP addresses. Instead, they come from a large range of non-globally routable IP addresses. When we replace an OIT Static IP Address with a recyclenet IP address, we are recycling a valuable resource and replacing it with one of less value.

Some may wonder why we replace a device's OIT Static IP address with one on recyclenet, instead of simply deleting the IP address or the entire Host Database entry.

We do not delete the entire Host Database entry for several reasons:

We replace the old IP address with one on recyclenet instead of removing the IP address from the entry for several reasons:


Appendix: Why is there no online way to "renew" an unused OIT Static IP Address?

After receiving a SIRF reminder that an IP address has been unused for a long time and may soon be recycled, customers sometimes ask why they may not "renew" their device's IP address assignment via an online form.

We do not provide such a facility because it makes it too easy to defeat the purpose of SIRF. If the IP address has been used for a long time, it doesn't need to remain reserved for the use of the customer's device.

Providing a self-service form a customer could use to prevent the unused IP address from being recycled would make it too easy for customers to continue to reserve these unused IP addresses.

Additionally, our experience is that nearly all customers who believe we must not recycle a device's unused IP address have misunderstood what's going on. Most often they mistakenly believe that the IP address has been in-use when it has not. We do not wish to make it easy for such customers to continue to hold onto IP addresses they are not using.

We do provide one straightforward way to "renew" an unused OIT Static IP Address: simply reconnect your device to the campus network in such a way that it uses the IP address prior to recycling day. You need only do so momentarily; that will result in our recording the IP address is "in use."


If You Need Additional Assistance

If you have questions or need assistance with any of the procedures in this document, please contact the OIT Support and Operations Center.



A service of OIT Network Systems
The Office of Information Technology,
Princeton University
Last Updated: September 11 2014