Complex sentence: In particular, all types of NAT break the originally envisioned model of IP end-to-end connectivity across the Internet and NAPT makes it difficult for systems behind a NAT to accept incoming communications.
Split into simpler sentences: In particular, all types of NAT break the originally envisioned model of IP end-to-end connectivity across the Internet. Network Address and Port Translation (NAPT) make it difficult for systems behind a NAT to accept incoming communications.