I am struggling to understand what the following / after an IP address (not .0 or .255) is telling me.

This is from MS's website:

Outlook.com Outbound IP Space
Outlook.com sends e-mail from the following IP addresses:
* 65.54.190.0/26
* 65.54.190.64/26
* 65.54.190.128/26
* 65.54.190.192/26

Is the /26 of any relevance in this case? If it is, what is it telling me?

If anyone can clarify I would be v grateful.

Search the forum for ip cheat sheet.

I couldnt find the cheat sheet, but if it's like all the other CIDR helpers out there it won't answer my question:

If you have an IP of 65.54.190.64/26, what is the relevance of /26 - why would I ever need to know more than just the IP?

It defines the subnet, address range, multicast address. It has nothing with mikrotik it is standard. Have you tried to google about at least?

You did not your homework well. Otherwise you would have find this:
http://forum.mikrotik.com/viewtopic.php ... 4&p=418752

Or use google, it is everywhere.

If you have 65.54.190.64/26, it means that it is the network address. You can use IPs from range 65.54.190.65-65.54.190.126, have netmask 255.255.255.192 and 65.54.190.127 is broadcast address.

I am struggling to understand what the following / after an IP address (not .0 or .255) is telling me.

This is from MS's website:

Outlook.com Outbound IP Space
Outlook.com sends e-mail from the following IP addresses:
* 65.54.190.0/26
* 65.54.190.64/26
* 65.54.190.128/26
* 65.54.190.192/26

Is the /26 of any relevance in this case? If it is, what is it telling me?

If anyone can clarify I would be v grateful.

These are the subnets of your network
65.54.190.0(broadcast ID) 65.54.190.1 - 62( IP Range) 65.54.190.63(Network ID)
255.255.255.192 will your subnet mask
Meaning in this network your have only 62 IPs to use
65.54.190.64 and the rests are in different network

I am struggling to understand what the following / after an IP address (not .0 or .255) is telling me.

This is from MS's website:

Outlook.com Outbound IP Space
Outlook.com sends e-mail from the following IP addresses:
* 65.54.190.0/26
* 65.54.190.64/26
* 65.54.190.128/26
* 65.54.190.192/26

Is the /26 of any relevance in this case? If it is, what is it telling me?

If anyone can clarify I would be v grateful.

65.54.190.0(broadcast ID) 65.54.190.1 - 62(IP Range) 65.54.190.63(Network ID)
meaning you have 62 available IPs in this network and the subnet mask is 255.255.255.192
65.54.190.64 and the rest are another network

To be clear here - it MS say that email may come from the 65.54.190.64/26 block then you should assume that it means from any of the IPs from .64 to .127 . The restriction on the use of .64 and .127 only occurs if the block is applied to a broadcast network such as Ethernet. In other situations all the addresses may be used.