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The limit on the number of NICs that can be attached to an instance, at least when using the Libvirt/KVM driver, is tied to the number of devices that the VM can have on a single PCI bus (32). Several of these slots are taken by system devices by default leaving you with 28 slots for NIC attachment [1].

Whether those NICs are on the same network or not is not a factor, if the environment is configured to allow multiple NICs on the same subnet (supported as of Juno) there is no explicit limit on how many there can be - the limit is the size of the addressable network and the aforementioned PCI bus.

[1] https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Host_Configuration_and_Guest_Installation_Guide/chap-Virtualization_Host_Configuration_and_Guest_Installation_Guide-Virtualization_Restrictions.html#sect-chap-Virtualization_Host_Configuration_and_Guest_Installation_Guide-Virtualization_Restrictions-KVM_Restrictions

The limit on the number of NICs that can be attached to an instance, at least when using the Libvirt/KVM driver, is tied to the number of devices that the VM can have on a single PCI bus (32). Several of these slots are taken by system devices by default leaving you with 28 slots for NIC attachment [1].

Whether those NICs are on the same network or not is not a factor, if the environment is configured to allow multiple NICs on the same subnet (supported as of Juno) there is no explicit limit on how many there can be - the limit is the size of the addressable network and the aforementioned PCI bus.bus limitation.

[1] https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Host_Configuration_and_Guest_Installation_Guide/chap-Virtualization_Host_Configuration_and_Guest_Installation_Guide-Virtualization_Restrictions.html#sect-chap-Virtualization_Host_Configuration_and_Guest_Installation_Guide-Virtualization_Restrictions-KVM_Restrictions