Locating Orphaned subnets in IPAM

Locating Orphaned IPAM Subnets

One of the key features of good IPAM is ability quickly find subnets that are part of the defined address space but have not been explicitly added to list of subnets available for allocation.

These subnets are normally called “orphaned” and can be presented as white spaces within address space.

Orphaned subnets are normally occurring when you import IPAM subnets from external source such as IGP routing table where address space is not contiguously divided among all the existing subnets.

Lets looks at this simplified example of the logic required for locating orphaned subnets.

For example, user defined full address space as 10.0.0.0/8 (10.0.0.0 – 10.255.255.255)

and imported one subnet from IGP: 10.20.20.0/24 (10.20.20.0 – 10.20.20.255)

We can present full address space as a contiguous line starting from 10.0.0.0 and ending 10.255.255.255

Full IPv4 Address Space:

10.0.0.0 ———————————————————————————-10.255.255.255

Now let’s overlay single imported subnet into address space line:

10.0.0.0 ————-***Used*** —————————————————10.255.255.255

We see that there is some unused space to the left and to the right of the used space.

We need to find all the subnets that cover unused space so they can be presented in IPAM list of available subnets. The subnets that we are looking for must be largest subnets possible,

to minimize fragmentation.

Let’s summarize: Our goal is to find the largest subnet(s) that can be fit into the left and right empty segments.

Let first look at “left” empty space:

We can see that First and Last IP address of “left” empty segment have matching highest 11 bits.

Largest subnet that will fit into “left” empty segment will have First and Last IP with following parameters:

First IP address:

1. Has 00001010‬.000 in first 11 bits
2. Has all zeros in Host portion
3. Is greater or equal to 10.0.0.0

Last IP address:

1. Has 00001010‬.000 in first 11 bits
2. Has all ones in Host portion
3. Is less or equal to 10.20.19.255

After checking all possible options matching IP addresses are discovered.

This combination of First/Last IP gives us largest Subnet that will fit into “left” free segment as 10.0.0.0/12

We can see that 10.0.0.0/12 does not fully cover “left” empty segment so this discovery process has to be repeated for remaining empty space. Discovery may require several iterations until we get 100% coverage.

Similar approach must be applied to “right” empty segment until we identify all orphaned subnets and achieve 100% coverage of required address Space.

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