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How to find if a subnet is available for allocation

wyko.ter.haar

I'm trying to programmatically create subnets and have run across the following challenge: How do you determine if a subnet is already in use?

Here's a snippet from my code, which gets subnet attributes from the database:

# Get the subnet attributes
q = swis.query("SELECT TOP 1 " \
    "SubnetId, Address, CIDR, Comments, VLAN, Location, Description, URI " \
    "FROM IPAM.Subnet "\
    "WHERE (Address = '{}' AND CIDR = {})".format(
        net.network_address,
        net.prefixlen
        )

If this returns an empty result set, then we know that the subnet does not exist, which is great. But say I have a subnet allocated already on 10.1.1.0/24, and I ask IPAM if 10.1.1.0/28 is available? It will tell me that the 28 does not exist, which is not what I want.

Any ideas?

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Python

Accepted answers

wyko.ter.haar

You got me on the right track, and I found a simple and fast solution! It looks like IPAM.IPNode has a field called IPAddressN, which is the IP address formatted into a weird hexadecimal-like field. Solarwinds SQL supports a simple greater-than and less-than lookup based on this field. For everyone else out there wanting the same thing, here's the Pythonic way to finding if a subnet is available!

def subnetAvailable(net: ip.ip_network) -> bool:
    '''Checks whether a given subnet is empty and available.
   
    It work sby searching for all the ip addresses in the given subnet and
    finding if any are allocated.
   
    Args:
        net (ip.ip_network): A subnet 

    Returns:
        bool: True if the subnet is empty and available
            for allocation.'''
   

    # Check all IP addresses within the new subnet to see if any of the IPs
    # already have a SubnetID assigned to them
    a = swis.query("SELECT TOP 1 SubnetID, IPAddress FROM IPAM.IPNode WHERE "\
                  "(NOT SubnetID is Null) AND "\
                  "IPAddressN >= '{}' AND "\
                  "IPAddressN <= '{}'".format(
                      IPtoGUUID(net.network_address),
                      IPtoGUUID(net.broadcast_address)
                  )
                  )['results']
   
    # If any IP addresses were returned, that means some part of the subnet
    # is unavailable for allocation. If no IP's in the given range have a
    # SubnetID, then the entire subnet is available.
    if len(a) == 0: return True
    else: return False

def IPtoGUUID(ip: ip.ip_address) -> str:
    '''Converts an IPv4 address to a Solarwinds-style GUUID as used in IPAddressN'''
   
    ip = ip.exploded.split('.')
    return '{:02X}{:02X}{:02X}{:02X}-0000-0000-0000-000000000000'.format(*map(
        int, list(reversed(ip)))).lower()

All comments

tdanner

To answer that question by querying for subnets, you would have to loop through and check each enclosing subnet (all the way out to /0, I guess!). But if you query for addresses, you could just check any address within the subnet you are trying to test. If it exists (doesn't matter whether it is available, reserved, etc. - only if IPAM has a record for it) then it can be returned *with a SubnetId* that you can use to see what subnet owns it.

wyko.ter.haar

That is a really great idea, and it gives a solid start to the question!

But let's suppose instead we want to allocate 10.1.0.0/16 and I have already created 10.1.255.240/28. With your idea, unless I pick an IP address in that one /28 I made, it'll show the subnet as being available again. I would need to run a select statement covering every IP address in the to-be-created subnet, and I can't think of a an easy and non-resource intensive way to do that off the top of my head.

tdanner

That's a tricky one. I believe there may be a way to issue a range query, where you would ask if there are any subnets >= 10.1.0.0 and <= 10.1.255.255. I sent this thread to an IPAM expert in case he can suggest something.

tomiannelli

Perhaps this way, this isn't the syntax but the logic of it for IPv4. Take the network of interest and break it down so you get an IP address in every possible subnet. Since there are many tools for subnet calculation the algorithm should be available some where. So for a 16 bit mask you would generate 32,768 ip addresses, one in each of the /31 subnets possible. It takes seconds to generate them all using SolarWInds' Advance subnet calculator. This is roughly half the number of IPs in that 16 bit.

Then execute your query to see if there is a match.

That is kind of a brute force approach. I don't always like them but sometimes they are easier and less trouble then coming up with an elegant binary comparison logic solution I think would be necessary otherwise.

wyko.ter.haar

def subnetAvailable(net: ip.ip_network) -> bool:
    '''Checks whether a given subnet is empty and available.
   
    It work sby searching for all the ip addresses in the given subnet and
    finding if any are allocated.
   
    Args:
        net (ip.ip_network): A subnet 

    Returns:
        bool: True if the subnet is empty and available
            for allocation.'''
   

    # Check all IP addresses within the new subnet to see if any of the IPs
    # already have a SubnetID assigned to them
    a = swis.query("SELECT TOP 1 SubnetID, IPAddress FROM IPAM.IPNode WHERE "\
                  "(NOT SubnetID is Null) AND "\
                  "IPAddressN >= '{}' AND "\
                  "IPAddressN <= '{}'".format(
                      IPtoGUUID(net.network_address),
                      IPtoGUUID(net.broadcast_address)
                  )
                  )['results']
   
    # If any IP addresses were returned, that means some part of the subnet
    # is unavailable for allocation. If no IP's in the given range have a
    # SubnetID, then the entire subnet is available.
    if len(a) == 0: return True
    else: return False

def IPtoGUUID(ip: ip.ip_address) -> str:
    '''Converts an IPv4 address to a Solarwinds-style GUUID as used in IPAddressN'''
   
    ip = ip.exploded.split('.')
    return '{:02X}{:02X}{:02X}{:02X}-0000-0000-0000-000000000000'.format(*map(
        int, list(reversed(ip)))).lower()

mesverrum

Maybe I'm not seeing it, but tdanner's original solution seems like it would still work as long as you are making sure to actually allocate addresses to any large subnets that you really use. Addresses automatically allocate for any /21 or smaller so you only need to take an extra step for bigger subnets that are actually in use. I can't find the link off the top of my head but I know there is that IPAM tool you could download to populate ip addresses in bulk as well as a way to do them in the GUI if you only have a few subnets you intend to track that are larger than /21.

Assuming that your large parent subnet is populated then a query for any ip within the desired subnet would come back with the result so you know you can't use it.

wyko.ter.haar

The problem with that first idea is when you try to create a subnet bigger than the ones you created before. In that situation, they can be some IP addresses which were not allocated to one of the smaller subnets. If you search for one of those they will return as available, even though there are other used IP addresses and subnets within your to-be-created subnet.