Switch MS120 does not support configuring a port with network /31?

[iagoaas]

Switch MS120 does not support configuring a port with network /31?

I am trying to configure an MS120 port with a mask of 254, but I get the message below.

ERROR: Static IP should not equal broacast, gateway must be valid IP address.

[alemabrahao]

This won't work, /31 is a unique address and you need at least 2 valid IPs (host and gateway). Are you sure it's this mask?

This is networking basics.

https://www.techtarget.com/searchnetworking/tip/IP-addressing-and-subnetting-Calculate-a-subnet-mask...

[KarstenI]

Regarding networking basics ... /31 is allowed for over 20 years on P2P links. But I am not sure the MX supports it. At least the ASA can handle it.

[jbright]

The MX can support /31 on the WAN Link. I have set one up and it does work.

[KarstenI]

Good to know!

[GIdenJoe]

You're no configuring a port but the management address of the switch.  You can't have L3 interfaces on an MS120.
It would also not make sense to have a management VLAN with a /31.

If you would however have an MS2x or higher you can create VLAN interfaces and those will need to have a /30 or less.

[iagoaas]

OK, got it, thanks for the feedback.

Actually my internet provider is giving me a /31 network. with that I'm trying to put an MS120 before two MX to get a balance.

[alemabrahao]

I think it's not a /31, It's probably a /30. For Warm spare you need at least a /29 or you need to put it behind a NAT.

[GIdenJoe]

Oh yes that is a typical one.  If you have a public subnet behind the ISP router you will need to run a /29 if you want a primary/standby firewall.  You only need to configure the external vlan on your switches.

The other possibility is just to have the ISP run a private subnet behind their router like the typical 192.168.0.0/24 and you use 192.168.0.2 as virtual IP on your HA pair and have the ISP port forward every incoming flow to that IP.