A simple understanding of DHCP flow

[Tudor]

Even if a device has a static address in it there is still a flow of DHCP requests.

Feb 12 17:25:07 dnsmasq-dhcp[5220]: DHCPDISCOVER(br0) **:**:**:**:**:**
Feb 12 17:25:08 dnsmasq-dhcp[5220]: DHCPACK(br0) 192.168.0.15 **:**:**:**:**:** device-name

If the device does not have a static address, the flow is longer.

Feb 12 17:25:07 dnsmasq-dhcp[5220]: DHCPDISCOVER(br0) **:**:**:**:**:**
Feb 12 17:25:07 dnsmasq-dhcp[5220]: DHCPOFFER(br0) 192.168.0.15 **:**:**:**:**:**
Feb 12 17:25:08 dnsmasq-dhcp[5220]: DHCPREQUEST(br0) 192.168.0.15 **:**:**:**:**:**
Feb 12 17:25:08 dnsmasq-dhcp[5220]: DHCPACK(br0) 192.168.0.15 **:**:**:**:**:** device-name

If the address is already allocated there will be a DHCPNACK from the server to the device.

[Andrew-G]

OK, but what does that mean in practical everyday terms for the rest of us?

[Tudor]

Just helps you understand what’s happening. Ignore it if you think it’s irrelevant.

[Andrew-G]

Don't get me wrong, I'm all for sharing knowledge, but there's still a "so what?" question.  You think it's relevant, I don't know whether it is or not as you haven't explained that bit.

Is there anything eg in hub or router settings that can be changed to beneficially apply this knowledge?

[Roger_Gooner]

Does this depend on whether the static IP addresses are in the scope (reservation range) of the DHCP server as some network administrators include the static IPs in the scope and then manually mark them as excluded or unavailable to the DHCP service for leasing. I don't like this as I prefer that the DHCP server should only be aware of the IP addresses which it assigns, meaning that there are two IP address ranges: one for static and one for DHCP reservation.

[Tudor]

I think it depends on the router. My UDM-Pro expects them outside the DHCP range which I think is the best option. No idea about the VM hubs as I have never run any of them in router mode.

[Client62]

Using the Hub 3 in Router mode..

The MACs of 2 devices (Cisco ATA & Epson printer) are added to the DHCP Reserved list these have IPs specified below the DHCP range.

I also have 2 more non DHCP devices (WiFi APs) with static IPs these are manually configured IPs again below the DHCP range.

[Roger_Gooner]

I don't understand what you're saying as surely the IP addresses of the Cisco ATA & Epson printer should be in the DHCP reserved range.

[Tudor]

My TV box has a reserved/preferred DHCP address way outside of the DHCP ‘window’ because the box does not allow a static address to be assigned as do all my Apple HomePods. Also I just get a DHCPREQUEST and DHCPACK for these devices.

[Roger_Gooner]

I don't understand why your router cannot set an IP address within scope for your TV box as this is just a basic feature. In any case I also don't understand why this box needs a fixed IP address when dynamic addessing works perfectly well in millions of other cases.