我有一台 PC 和一台笔记本电脑连接到本地网络。两者都有 Ubuntu 18.04.4 桌面版，并且两者都对调制解调器可见：
但是当我尝试从 PC ping 笔记本电脑时，它显示Destination Host Unreachable：:
(base) marco@pc:~$ sudo ping 192.168.1.9
PING 192.168.1.9 (192.168.1.9) 56(84) bytes of data.
From 192.168.1.7 icmp_seq=1 Destination Host Unreachable
From 192.168.1.7 icmp_seq=2 Destination Host Unreachable
笔记本电脑上没有活动的防火墙：
(base) marco@marco-U365G:~$ sudo ufw status
Status: inactive
这是笔记本电脑的 /etc/netplan/01-network-manager-all.yaml：
network
version: 2
renderer: NetworkManager
# renderer: networkd
ethernets:
enp3s0:
dhcp4: no
addresses: [192.168.1.9/24]
gateway4: 192.168.1.1
nameservers:
addresses: [8.8.8.8,8.8.4.4]
我尝试在 PC 和笔记本电脑上使用 nmap：
(base) marco@pc:~$ sudo nmap -sn 192.168.1.0/24
Starting Nmap 7.60 ( https://nmap.org ) at 2021-05-06 18:28 CEST
Nmap scan report for www.adsl.vf (192.168.1.1)
Host is up (0.00045s latency).
MAC Address: 74:36:6D:9A:5A:63 (Unknown)
Nmap scan report for pc (192.168.1.7)
Host is up.
Nmap done: 256 IP addresses (2 hosts up) scanned in 3.77 seconds
从笔记本电脑来看：
(base) marco@marco-U365G:~$ nmap -sn 192.168.1.9/24
Starting Nmap 7.60 (https://nmap.org ) at 2021-05-06 18:29 CEST
Nmap scan report for_gateway (192.168.1.1)
Host is uo (0.00075s latency)
Nmap scan report for marco-U365C (192.168.1.9)
Host is uo (0.000054s latency)
Nmap done: 256 IP addresses (2 hosts up) scanned in 3.21 seconds
在笔记本电脑中，我还 ping 了本地主机，以检查网络接口是否启动：
(base) marco@marco-U354G:~$ oung 127.0.0.1
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.094 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.085 ms
实际上，我意识到 ping 其他设备也不起作用：
(base) marco@pc:~$ sudo ping 192.168.1.3 PING 192.168.1.3 (192.168.1.3) 56(84) 字节数据。来自 192.168.1.7 icmp_seq=1 目标主机不可达
我意识到这种情况正在发生，可能是因为互联网服务提供商更改了路由器。
如何解决这个问题？