fix(ipam): add unreservable ip info (#3800)

Author: RoRoJ

network/ipam/concepts.mdx

@@ -45,6 +45,8 @@ Private IP addresses identify devices on Private Networks. They are not routed o
 
 When you attach a resource to a Private Network, you can either specify a [reserved IP address](/network/ipam/how-to/reserve-ip/) to use for the attachment, or let the Private Network's [DHCP](/network/vpc/concepts/#dhcp) assign it a private [IPv4](#ipv4) address from the Private Network's [CIDR](/network/vpc/concepts/#cidr-block) block. In both cases, the resource's private IP address will not change unless you detach the resource from the Private Network, or delete the resource. [IPv6](#ipv6) addresses can also be assigned for IPv6-compatible resources.
 
+Four IP addresses from each CIDR block are unavailable for reservation/use: the first two and last two in the block. For example, for the subnet `172.16.12.0/22` the following addresses cannot be reserved or assigned to any resource: `172.16.12.0`, `172.16.12.1`, `172.16.15.254` and `172.16.15.256`.
+
 ## Public (flexible) IP address
 
 Public IP addresses identify devices on the public internet. You can enter the public IP address of an Instance into any browser connected to the Internet, and access content being served from that Instance. 

network/ipam/how-to/reserve-ip.mdx

@@ -46,6 +46,10 @@ This page sets out the steps necessary to reserve an IP address with IPAM.
 
 4. Define whether you want to specify the IP address to reserve, or reserve any available IP on the Private Network (the IP selected will be displayed after reservation). If specifying the IP address yourself, do **not** include the network prefix (e.g. `/22`).
 
+    <Message type="tip">
+    Four IP addresses from each CIDR block are unavailable for reservation: the first two and last two in the block. For example, for the subnet `172.16.12.0/22` the following addresses cannot be reserved or assigned to any resource: `172.16.12.0`, `172.16.12.1`, `172.16.15.254` and `172.16.15.256`.
+    </Message>
+
     The IP address is reserved, and you are returned to the list of your private IP addresses, where the reserved address now displays. You can use this address to attach a resource to the Private Network you reserved it from. If and when you no longer need the reserved IP, you can [release](#how-to-release-a-reserved-private-ip-address) it.
 
 ## How to attach a resource to a Private Network using a reserved IP address

network/ipam/quickstart.mdx

@@ -76,4 +76,8 @@ You can currently use IPAM in the Scaleway console to list the [private IP addre
 
 4. Define whether you want to specify the IP address to reserve, or reserve any available IP on the Private Network (the IP selected will be displayed after reservation). If specifying the IP address yourself, do **not** include the network prefix (e.g. `/22`).
 
+    <Message type="tip">
+    Four IP addresses from each CIDR block are unavailable for reservation: the first two and last two in the block. For example, for the subnet `172.16.12.0/22` the following addresses cannot be reserved or assigned to any resource: `172.16.12.0`, `172.16.12.1`, `172.16.15.254` and `172.16.15.256`.
+    </Message>
+
     The IP address is reserved, and you are returned to the list of your private IP addresses, where the reserved address now displays. You can use this address to attach a resource to the Private Network you reserved it from. If and when you no longer need the reserved IP, you can [release](/network/ipam/how-to/reserve-ip/#how-to-release-a-reserved-private-ip-address) it.