best practice for fixed-address / reserved leases

[Michael Schnyder]

Hi

Need to have an answer for correct configuring / assigning fixed ip addresses:

We defined a while ago our "best practice" to configure a fixed address definition outside the dhcp range:

-------------- example
subnet 172.1.2.0 netmask 255.255.255.0 {
  range 172.1.2.10 172.1.2.50;
  option routers 172.1.1.1;
  default-lease-time 21600;
}
# Fixed Address 1
host abc {
  hardware ethernet ab:c2:33:22:66:44;
  fixed-address 172.1.2.51;    ### <- ip is NOT within the range 172.1.2.10 172.1.2.50;
}
# Phone Fixed Address
host cde {
  hardware ethernet cd:e2:33:22:66:44;
  fixed-address 172.1.2.52;    ### <- ip is NOT within the range 172.1.2.10 172.1.2.50;
}
------------- example end

For me it's unclear why we should define it like that and unfortunately, I could not find yet an answer in the manual (also see [1]), therefore my question:

Is it mandatory that a "fixed" ip address is part of the range?
Or is it mandatory that it is _explicitly not_ part of the defined range?
Or does it matter at all? Can I define within the subnet whatever ip address I would like to?


[1] https://kb.isc.org/docs/isc-dhcp-41-manual-pages-dhcpdconf#RESERVED%20LEASES

Thanks for your tipps.

Kind regards
michael