I'm using ESP8266's to create time controlled switches and I can't get an IPAddress to work with WiFi.hostByName().When the ESP units try to get an ntp Server address, all IPAddresses I try work except 192.168.1.243 which fails with "(IP unset)" and shows no network activity.

The problem seems to occur when Wifi.config(), WiFi.hostByName() and possibly WiFi.hostname() are used together.Any ideas about why this one address doesn't work?

Notes:

Using win10-1903-18362

Ideas tried that didn't work:
1. Flush the DNS cache.
2. Restart the router.
3. Clean out the hosts file.
4. Check the other devices an see that they don't use the address.
5. Verify there's no reference to the address in the registry.
6. Use a utility to search the network to see if the address is already online elsewhere.

This is the simplified code:
#include <ESP8266WiFi.h>
#include <WiFiUdp.h>
#include <stdio.h>
#include <string.h>

IPAddress hostIP = IPAddress(192, 168, 1, 243);
IPAddress gateway = IPAddress(192, 168, 1, 1);
IPAddress subnet = IPAddress(255, 255, 255, 0));

void setup()
{
Serial.begin(115200);
while(!Serial);
WiFiUDP Udp;
WiFiServer server(80);

Serial.print("\nConnecting to ");
Serial.println(ssid);
WiFi.hostname("ESP8266_243");
WiFi.begin(ssid, password);
while (WiFi.status() != WL_CONNECTED)
{
delay(500);
Serial.print(".");
}
Serial.println("\nWiFi connected.");

WiFi.config(hostIP, gateway, subnet);
Udp.begin(4333);
IPAddress ntpServerIP;
WiFi.hostByName("us.pool.ntp.org", ntpServerIP);
} //Setup

void Loop()
{
}

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