This document discusses using 31-bit prefixes on IPv4 point-to-point links to more efficiently use limited IPv4 address space. It describes how 31-bit prefixes allow creating two point-to-point links using the same IPv4 resource that would traditionally support only one link. The document summarizes configuration of 31-bit prefixes on various router platforms like Cisco, Juniper, MikroTik, and challenges with platforms that do not natively support the RFC specification for 31-bit prefixes. It provides workarounds used to implement 31-bit prefixes on older equipment through configuration improvements and adjustments.

1. 31
Get more from your IPv4 resources
Presented By:
Shuvodip Roy
Senior Systems
Engineer
Amber IT Limited
sdroy@live.com

2. Preface
As IPv4 exhausted, RIR’s (Regional Internet Registry) are not getting IPv4 to
be allocated or the block size now become very limited. Alternative and
the next version IPv6 is rolled out. But, demand of IPv4 is still growing.
One of the big reason is globally still the maximum number of sites yet
only available via IPv4.
In the context of Bangladesh, we still need to connect a large numbers of
subscribers with Internet. Sufficient IPv4 addresses are not available. In
this circumstance, we can use RFC3021 Using 31-Bit Prefixes on IPv4
Point-to-Point Links in large scale.
In a ISP or else, 80% or more are actually “Single Home Point-to-Point
Links”. Using the RFC3021, we will be able to connect double numbers of
links than current times using existing IPv4 resources.
This presentation is all about that.
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3. Disclaimer
* Disclaimer: Although RFC3021 released in December, 2000. Still many
vendors or devices not support it. In this presentation, will try to cover up
the configuration of those which is commonly use in Bangladesh.
The configuration will differ vendor to vendor as well as device to device.
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4. Exhaustion
All 5 RIR's have either limited supplies or have run out of available IPv4
addresses. ARIN and the RIPE NCC ran out of their available supply in 2015
and 2019 respectively. In APNIC, LACNIC and AFRINIC rationing supplies
according to the community policies.
APNIC’s IPv4 pool to run out by creating policies to limit the amount of
address space delegated to APNIC Members from APNIC’s final /8.
Implemented the final /8 policy, which stipulated that APNIC account
holders new or existing were able to receive up to a /23 from the final /8
pool.
Checkout of the report of IPv4 availability:
https://labs.apnic.net/ipv4/report.html
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5. Request for Comments: 3021
In December, 2000 the documentation open. The main motivation was to
relief the problem slightly of lack of IPv4 addresses. On that time IPv6 was not
in production. Still IPv6 required many more years for transition fully.
Key Points:
• Consideration of 31-Bit Prefixes on IPv4 Links.
• Networks with 31-bit prefixes have no impact on current routing protocols.
The communication between peers is done using multicast, limited
broadcast or unicast addresses (all on the local network), none of which are
affected with the use of 31-bit subnet masks.
• The use of 31-bit subnet masks within the core of the Internet will reduce
the number of physical links against which a DoS attack relying on packet
replication through the use of directed broadcasts can be launched
[RFC2644, SMURF]
More information available in: https://datatracker.ietf.org/doc/html/rfc3021
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6. 31-Bit Prefixes
Network: 192.0.2.0/31
Address: 192.0.2.0 11000000.00000000.00000010.0000000 0
Netmask: 255.255.255.254 = 31 11111111.11111111.11111111.1111111 0
Wildcard: 0.0.0.1 00000000.00000000.00000000.0000000 1
Broadcast: 192.0.2.1 11000000.00000000.00000010.0000000 1
HostMin: 192.0.2.1 11000000.00000000.00000010.0000000 1
HostMax: 192.0.2.0 11000000.00000000.00000010.0000000 0
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7. Traditional Point to Point Link
IP Block of 30 Bit Prefix: 192.0.2.0/30
Network Address: 192.0.2.0
Provider Edge Address: 192.0.2.1
Client Edge Address: 192.0.2.2
Broadcast Address: 192.0.2.3
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Provider Edge
IP: 192.0.2.1
Client Edge
IP: 192.0.2.2

8. Point to Point Link with 31-Bit
Using the same 192.0.2.0/30 rather than traditional Point to Point link in
31-Bit we can create 2 Point to Point link by same resource.
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Provider Edge
IP: 192.0.2.0
Client Edge
IP: 192.0.2.1
Provider Edge
IP: 192.0.2.2
Provider Edge
IP: 192.0.2.3
Client 1 Point to Point Link
Client 2 Point to Point Link
Client 1
/31 Bit Prefix:
192.0.2.0/31
Network Address:
192.0.2.0
Provider Edge Address:
192.0.2.1
Client Edge Address:
192.0.2.2
Broadcast Address:
192.0.2.1
Client 2
/31 Bit Prefix:
192.0.2.2/31
Network Address:
192.0.2.2
Provider Edge Address:
192.0.2.2
Client Edge Address:
192.0.2.3
Broadcast Address:
192.0.2.3

9. Where 31-Bit can be use?
In a ISP:
• Point to Point Infrastructure Link (i.e.: Backbone) with Public IP.
• Point to Point connected link with “Single Home” dedicated corporate
customers.
• It is also applicable for those customers who have multiple or single link
and the Public IP block routed via any Routing Protocol. Etc.
In a Organization:
• Point to Point Internal Backbone with Public IP.
• In Host (i.e.: Servers, Workstation) running with 30-Bit Prefix. Etc.
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10. Challenges!!!
A number of vendors or Operating System still now not support the
RFC3021. As a fact, a significant challenges is there to implement.
* In the context of Bangladesh, CISCO IOS support this since release
12.2(2)T, Juniper supports it also.
* Mostly used MikroTik, does not support it but with improvisation
without any side effects it possible to configure with 31-Bit.
* Windows is same as Mikrotik.
* Home based Router TP-Link new version supports 31-Bit but for older
versions need improvisation. But, that improvisation have relative side
effects also. Which will be discussed later to be concerned to use.
* Others all depends on Vendors and it’s Firmware or OS.
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11. Point to Point with 31-Bit in
CISCO
For R1:
R1(config-if)#ip address 192.0.2.0 255.255.255.254
For R2:
R2(config-if)#ip address 192.0.2.1 255.255.255.254
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Host Name “R1”
IP: 192.0.2.0/31
Host Name “R2”
IP: 192.0.2.1/31

12. Point to Point with 31-Bit in
CISCO (continue)
After configure IP in both Router if we check Route Table and Reachability
via ping, we will get following.
As CISCO supported RFC3021 31-Bit Prefixes will operate smoothly.
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R1 R2

13. Point to Point with 31-Bit in
Juniper
For R1:
[edit interfaces fxp0 unit 0]
root@R1# set family inet address 192.0.2.0/31
For R2:
[edit interfaces fxp0 unit 0]
root@R2# set family inet address 192.0.2.1/31
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Host Name “R1”
IP: 192.0.2.0/31
Host Name “R2”
IP: 192.0.2.1/31

14. Point to Point with 31-Bit in
Juniper (continue)
After configure IP in both Router if we check Route Table and Reachability via
ping, we will get following.
Juniper also supported RFC3021 31-Bit.
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R1 R2

15. Point to Point with 31-Bit in
MikroTik
For R1:
[admin@R1] > ip address add address=192.0.2.0 network=192.0.2.1
interface=ether1
For R2:
[admin@R2] > ip address add address=192.0.2.1 network=192.0.2.0
interface=ether1
Note: Mikrotik does not support the RFC3021. In MikroTik need to declare IP
as 32-Bit and also need to add “network” manually.
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Identity “R1”
IP: 192.0.2.0/31
Identity “R2”
IP: 192.0.2.1/31

16. Point to Point with 31-Bit in
Mikrotik (continue)
After configure IP in both Router if we check Route Table and Reachability
via ping, we will get following.
Now in Mikrotik we have the Reachability.
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R1 R2

17. Issues with MikroTik
We already ensure the Reachability in Point To Point link in Mikrotik by
using improvisation using 31-Bit as 32-Bit.
So, far in this method we can use 31-Bit Prefixes in Mikrotik. But, it creates
a issue also.
• Connected Route not Redistribute:
Suppose, Router R1 is a POP of a ISP and R2 is Client Edge. ISP POP is
connected over Dynamic Routing Protocol where Connected Route
Redistributed over Dynamic Routing Protocol.
As we manually set the IP as 32-Bit and also the network in R1. So that the
original network=192.0.2.0 changed for R1 IP. Which is manually set to
network=192.0.2.1. In that case, it will not Redistribute over Dynamic
Routing Protocol.
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18. Solution of the Issues with
MikroTik
There are 2 possible solution of this currently.
• Advertise the 31-Bit Prefix: Solution 1, will be the advertising 31-Bit
Prefix 192.0.2.0/31 via Dynamic Routing Protocol in POP/R1 side.
• Set the HostMin IP in POP/R1 as 31-Bit: Solution 2, is in POP/R1 declare
IP as 192.0.2.1/31 where automatically network will be 192.0.2.0. In
Client/R2 set the IP manually as 192.0.2.0 as well as network 192.0.2.1.
Thus Point to Point will be reachable also the connected route in
POP/R1 will be Redistribute also.
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19. Solution of the Issues with
MikroTik
There are 2 Possible solution of this currently.
• Advertise the 31-Bit Prefix: Solution 1, will be the advertising 31-Bit
Prefix 192.0.2.0/31 via Dynamic Routing Protocol in POP/R1 side.
• Set the HostMin IP in POP/R1 as 31-Bit: Solution 2, is in POP/R1 declare
IP as 192.0.2.1/31 where automatically network will be 192.0.2.0. In
Client/R2 set the IP manually as 192.0.2.0 as well as network 192.0.2.1.
Thus Point to Point will be reachable also the connected route in
POP/R1 will be Redistribute also.
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20. 31-Bit in TP-Link
TP-Link Routers are mostly used in Home, SOHO link. New firmware and
models TP-Link supports RFC3021 and configuration is nothing new.
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31-Bit IP configuration in TP-Link

21. Issues with TP-Link old
versions
Older version TP-Link does not support RFC3021. In that case, through
improvisation setup could be possible as figured.
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TP-Link 31-Bit configured with improvisation in old version

22. Issues with TP-Link old
versions (continue)
In the early slide we configured 31-Bit in a old version/model of TP-Link
which does not support the RFC3021.
In that case, Prefix was 118.179.84.12/31 but as TP-Link does not support
it Subnet Mask: 255.255.255.254 or 255.255.255.255 will not be
applicable. It will provide following error
“The IP Address does not match the Subnet Mask: Invalid host number”
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23. Solution of the Issues with
TP-Link old versions
In 118.179.84.12/31, we need to find a sweet able Subnet Mask where IP:
118.179.84.12 & IP: 118.179.84.13 will not be the Network Address or the
Broadcast Address.
So, in nearest Subnet Mask 255.255.255.248 or /29 IP: 118.179.84.12 &
IP: 118.179.84.13 is not the Network Address or the Broadcast Address.
Then Router will accept the IP configuration and 31-Bit will be useable.
The nearest Subnet Mask will vary with IP.
Caution: There also a side effect brings with this improvisation. As TP-
Link get 118.179.84.8/29 as connected Route due to Subnet Mask:
255.255.255.248. So, from this TP-Link except 118.179.84.12 & IP:
118.179.84.13 all IP will be unreachable in 118.179.84.8/29. If the link
required reachability in this range then this improvisation will be
difficult.
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24. Others Home Routers
Others Home Router such as Netgear, Asus, Tenda, D-Link etc. are
moreover same as TP-Link.
The new version in some vendors supports RFC3021 but others are not.
Improvisation works but there are side effects of that which already
described.
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25. 31-Bit in Windows
Windows also not support RFC3021. If we set the Subnet Mask
255.255.255.254 it provides following Warning.
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26. 31-Bit in Windows (continue)
But as improvisation we may use the IP as 32-Bit Prefix, that means
Subnet Mask is 255.255.255.255
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31-Bit in Windows

27. 31-Bit in Windows (continue)
Although it will also provide following Warning. But, saving this
configuration by pressing “Yes” will work for 31-Bit Prefix improvisation in
Windows. Thus, smoothly 31-Bit can be useable in Windows.
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28. 31-Bit in Linux
Adding 31-Bit Prefix is Linux have nothing new. It is usual as like others IP
configuration in Linux. Here is a example with Ubuntu 20.10 (Groovy
Gorilla)
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31-Bit Prefix in Linux

29. Example in Production
In my organization Amber IT Limited, we are deploying 31-Bit Prefixes in
respect of the concerns of all matters which already described. Plenty of
numbers we already deployed in 31-Bit and it’s cost IPv4 resources half
rather than where it traditionally does with 30-Bit Prefix.
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30. Summary
In Exhaustion, RFC3021 is very helpful in deed. There are no doubts IPv6 is
the longer time solution. But, different expertise already provide
statement that it will require more time to transition fully in to IPv6.
In the mean time, we have none other choice but a option RFC3021.
Which will provide a relief for Exhaustion of IPv4.
The main challenge is primeval Devices and many Vendors still not support
the RFC. As it is a matter of Firmware or OS, we may expect soon Vendors
will rollout updated Firmware or OS which will support RFC3021. Which
will be a great offer for users. It will be also a great relief for Service
Providers.
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31. Acknowledgement
Special thanks for bdNOG committee members for approve my
presentation.
I would also like to gratefully acknowledge who works and proposed
RFC3021.
• A. Retana
• R. White
• Cisco Systems
• V. Fuller
• GTE Internetworking
• D. McPherson
• Amber Networks
and also to many more.
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