Two tools are used for LAN IP survival and port detection: Ping and Telnet. Here, we will first discuss ICMP and application scenarios, and combine them with Telnet to achieve IP survival and port detection, in order to identify hosts that may be at risk of intrusion ICMP: ICMP is the abbreviation for Internet Control Message Protocol, which is the Internet Control Message Protocol. In a local area network environment, if you want to do a crawler program to analyze which IPs are occupied and sniff whether it has opened regular ports (such as HTTP port 80 and SSH port 22), the fastest and simplest way to detect survival is through ICMP packets. Of course, the prerequisite is that the opposing host allows ICMP packet requests and responses, otherwise the goal cannot be achieved. In general, most hosts allow ICMP message interaction by default, mainly used to detect host survival The difference between ICMP, TCP, and UDP: ICMP is not used for transmitting data between two points, but is typically used for analyzing routing, detecting network connectivity, and determining network latency. Except for the system's built-in ping and traceroute tools for detecting routes, they are generally not directly used by network programs. In IPv4, it corresponds to ICMPv4, while in IPv6, it corresponds to ICMPv6. TCP is used for reliable data connection transmission and requires both parties to establish a stable connection for data exchange; UDP is used in data transmission scenarios that lack reliability and connectivity. Both parties do not need to maintain stable connections, and data may be lost, incorrect, and duplicated. ICMP, like UDP protocol, is an unreliable protocol. It encapsulates ICMP headers directly on top of IP packets and is not based on TCP/UDP protocols