diff --git a/src/Year_2016/P7.py b/src/Year_2016/P7.py
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+++ b/src/Year_2016/P7.py
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+# --- Day 7: Internet Protocol Version 7 ---
+
+# While snooping around the local network of EBHQ, you compile a list of IP
+# addresses (they're IPv7, of course; IPv6 is much too limited). You'd like to
+# figure out which IPs support TLS (transport-layer snooping).
+
+# An IP supports TLS if it has an Autonomous Bridge Bypass Annotation, or ABBA.
+# An ABBA is any four-character sequence which consists of a pair of two
+# different characters followed by the reverse of that pair, such as xyyx or
+# abba. However, the IP also must not have an ABBA within any hypernet
+# sequences, which are contained by square brackets.
+
+# For example:
+
+#     abba[mnop]qrst supports TLS (abba outside square brackets).
+#     abcd[bddb]xyyx does not support TLS (bddb is within square brackets, even
+# though xyyx is outside square brackets).
+#     aaaa[qwer]tyui does not support TLS (aaaa is invalid; the interior
+# characters must be different).
+#     ioxxoj[asdfgh]zxcvbn supports TLS (oxxo is outside square brackets, even
+# though it's within a larger string).
+
+# How many IPs in your puzzle input support TLS?
+
+# from collections import deque
+# from itertools import islice #tee
+
+import re
+
+with open("files/P7.txt") as f:
+    IPs = [
+        re.split(r"\[([^\]]+)\]", line) for line in f.read().strip().split()
+    ]
+
+
+def abba(s: str) -> bool:
+    for i0, i1, i2, i3 in zip(s, s[1:], s[2:], s[3:]):
+        if i0 == i3 and i1 == i2 and i0 != i1:
+            return True
+    return False
+
+
+def part_1() -> None:
+    count = 0
+    for ip in IPs:
+        sn, hn = (" ".join(ip[::2]), " ".join(ip[1::2]))
+        if abba(sn) and not (abba(hn)):
+            count += 1
+    print(count)
+
+
+if __name__ == "__main__":
+    part_1()