Which practice is commonly used by ISPs to prevent spoofed source addresses from entering the network?

• A. Ingress filtering
• B. Egress filtering
• C. TCP Intercept
• D. Wavelet-Based Signal Analysis

Answer: (A)

Ingress filtering at the ISP edge is used to prevent spoofed source addresses from entering the network. It works by checking incoming packets to ensure their source IPs belong to networks that the ISP is authorized to route to that interface; if a packet has a source address that shouldn’t come from that connection, it is dropped. This helps stop IP spoofing at the entry point, reducing the effectiveness of spoofed-attacks and misdirected traffic. This practice is widely recommended in guidelines like BCP 38 for ISPs.

Egress filtering, by contrast, looks at outbound traffic and blocks packets with spoofed source addresses from leaving the network. TCP Intercept and Wavelet-Based Signal Analysis aren’t focused on preventing spoofed addresses entering the network.