I have a Pi3B connected to my router via Ethernet, and have no problem on my LAN- WiFi is OK.
In fact, I use it as a file and LAMP server using Raspbian STRETCH. But I recently noticed the clock was off by several minutes, so it was rebooted, and still it was wrong.
THEN- After a apt-get update didn't work, I noticed I couldn't even do a ping 126.96.36.199 ! It hadn't been connected to the internet for several months!
In spite of Apache, PHP and Samba running along OK!
I'm using a static IP (...200.99) instead of a DHCP address.
So what's the problem?
Here's a copy of my ifconfig- And note that there is no WLAN in it, so I assume it's connected to the router via Ethernet.
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 inet 192.168.200.99 netmask 255.255.255.0 broadcast 192.168.200.255 inet6 fe80::ba27:ebff:fe8f:a5bf prefixlen 64 scopeid 0x20<link> ether b8:27:eb:8f:a5:bf txqueuelen 1000 (Ethernet) RX packets 70004 bytes 13843226 (13.2 MiB) RX errors 0 dropped 2 overruns 0 frame 0 TX packets 2381 bytes 175770 (171.6 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0 lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536 inet 127.0.0.1 netmask 255.0.0.0 inet6 ::1 prefixlen 128 scopeid 0x10<host> loop txqueuelen 1000 (Local Loopback) RX packets 1495 bytes 150918 (147.3 KiB) RX errors 0 dropped 0 overruns 0 frame 0 TX packets 1495 bytes 150918 (147.3 KiB) TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0