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AS mentioned, used several IP scanners, or ways to view IP addresses used  with MAC. whether dynamic? or static?
Well....
each show different sets of IP and MAC, so don;t know which results to believe

For example:
ran "arp -a"
ISP DHCP client lists (I know ISP will list DHCP IP that was assigned for 24hr period, so not same list as "arp -a"
NetScan tools
Advanced IP Scanner
etc.....

ALL show different results. VERY CONFUSING

So it is easier to just avoid it all together and don't put statically assign hosts IPs inside the DHCP range.
-Otanx

TRUE, but at least here in Philippines ISP router is LOCKED from "Advanced" settings, so users like us can NOT access ISP router to assign DHCP IP ranges.  We can only access simple things changing SSID, password, etc...

What we've noticed is that ISP router don't seem to assign dynamic IP past 192.168.1.100, so all static IP have been using 192.168.1.100 and higher.

What is unknown is when internet is SLOW on devices, was it due to IP conflict? or bad(slow) ISP internet service?
MYSTERY...

Yes, I am learning that it's not a clear cut issue. Many factors are considered.
I ran several DHCP IP scanners, all show different dynamic and static IP lists.  I don;t know which to believe.
Lucky this is only a home network.  I was just curious.

I think I found my asnwer.

"The DHCP server typically attempts to verify whether an IP address is currently in use before assigning it to a client. However, there is no guarantee that the manually configured client is currently connected to the network or available to respond when the DHCP server is verifying the IP address. So, the DHCP server might assign the IP address to a DHCP client. When the manually configured client connects to the network, you will have duplicate IP addresses on your network. IP addresses that are managed by DHCP should not be used to manually configure the network setup for a client. If a client needs to be manually configured with an IP address, that IP address should be excluded from the DHCP server's address pool."

[same IP address]

I was reading following statement IP conflicts.

"Employing both static and dynamic IP address allocation to configure IP addresses for network devices can lead to IP conflicts. For instance, a network admin might have assigned the IP address 192.168.1.10 to a device A (static IP allocation), and a Dynamic Host Configuration Protocol (DHCP) server operating on that network might assign the same IP address to a requesting network device (dynamic IP allocation); this can cause two different devices to have the same IP address, causing IP conflict on network."

I thought IF I assigned STATIC IP to devices then DHCP server will not(can't) assign the same IP to other devices on network.
But above statement says otherwise..that DHCP can assign same IP(Static) to other devices.
So is this TRUE?

Here is my networking setup at home with many rooms that need wifi.
ISP router/modem, then (4)routers connected to ISP LAN1-3
ISP gateway 192.168.1.1. But room routers are 192.168.254.254. 192.168.0.1 (different from ISP 192.168.1.1)
Technically room routers DHCP should be disabled, but (2) room routers DHCP left enabled per factory setting.
Just for experiment, I gave all devices in rooms STATIC IP(192.168.1.xxx) with 192.168.1.1 as gateway and DNS1. 
This way don't matter IF room routers DHCP is disabled or not.
All devices got wifi and internet,  All HAPPY...

Then few WEIRD things happened..

First, all of sudden all devices can not get all websites. EX: can't get google or youtube but can get bank and speedtest websites. remember some room routers were still set to DHCP ENABLED. some disabled.
Then over night back to normal, access all websites,  I done NOTHING to network, except to got sleep.
Can anybody explain why some websites(google/youtube) were not available?
BTW even ISP router wifi connection did same(only some websites can connect)

Second, when connecting devices to ISP wifi (192.168.1.1), devices are getting 192.168.254.xxx or 192.168.0.xxx, something other than ISP gateway 192.168.1.xxx.
Is there some kind of feedback to ISP router from room routers with 192.168.254.xxx or 192.168.0.xxx ?
OR are devices (connected to ISP wifi) getting IP from whichever routers on network connects first?

The whole idea of testing wifi connection with STATIC IP on devices is to see IF DHCP really need to DHCP disabled always.

Any comments or discussions are appreciated