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Webcast-Catalyst9k
New Member

Public IP Subnetting and routing

Hi all,

I have a scenario whereby I have been assigned a public IP address space which I need to subnet into smaller networks (one per customer); for ease of understanding we will say I have been assigned a /24 range.  Most of my experience comes from working within the LAN therefore this is a unique situation for me.  To give you some insight into the network configuration, I have a layer 3 switch stack which is using layer 2 and layer 3 SVI VLANs to segment different customers, along with ACLs and QOS.  Beyond the switch stack are access level switches and customer firewalls performing NAT/PAT functionality to translate the assigned public ranges to private.  There is no requirement to route between VLANs, only to route to the internet (which is performed from the switch stack via its default route) which has been provided via an ISP.  My confusion comes into play with how the return routing will be handled.  Basically, I need to do the following -

  • Segment the /24 IP range provided by the ISP into smaller networks (/29 and possibly /30) per customer

My query is, if I am subnetting and assigning /29 and /30 subnets for the /24 range to customer VLANs, how would the return routing work because the ISP has assigned me a /24 range but my equipment will only know about the ranges I have assigned, and their equipment will only know about the /24 range.  I have tied my mind up in knots on this and im pretty sure it really isn't that difficult to perform and understand this!?!

Please save me from this mind limbo I have found myself in!

1 ACCEPTED SOLUTION

Accepted Solutions
VIP Super Bronze

Public IP Subnetting and routing

Hi Nick,

It will work fine.  The SP is providing a /24 and send everything to your /24.  From there, you take the /24 and break it down to multiple smaller subnet.  The /24 is an aggregate route for smaller subnets.  If for example the ISP would have given you a /22, you could divide that to multiple /24 (4). but all the ISP cares for is the /22. (aggregate route).

HTH

2 REPLIES
VIP Super Bronze

Public IP Subnetting and routing

Hi Nick,

It will work fine.  The SP is providing a /24 and send everything to your /24.  From there, you take the /24 and break it down to multiple smaller subnet.  The /24 is an aggregate route for smaller subnets.  If for example the ISP would have given you a /22, you could divide that to multiple /24 (4). but all the ISP cares for is the /22. (aggregate route).

HTH

New Member

Public IP Subnetting and routing

Reza,

Thanks for the reply, I am currently in the process of testing this and have performed the following to route the traffic to the internet -

  • 8 Customer VLANs each with an SVI
  • I have been assigned a /26 for the service provider (64 addresses)
  • I have assigned each customer with either a /29 or a /30 depending on service
  • Internet facing VLAN on /30

I have created an internet facing VLAN with a /30 which is in the same subnet as the upstream routing device.  I have then setup a default route which sends all default traffic to this upstream device.  I have then assigned each customer with either a /30 or /29 depending on what they have purchased.  This works successfully but basically I would like confirmation that this is indeed the best method of performing this type of routing or is there a "better" method?

Thanks

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