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We study correlation functions involving extended defect operators in the four-dimensional ${\cal N}=4$ super-Yang-Mills (SYM). The main tool is supersymmetric localization with respect to the supercharge $\cal Q$ introduced in \cite{Pestun:2009nn} which computes observables in the $\cal Q$-cohomology. We classify general defects of different codimensions in the ${\cal N}=4$ SYM that belong to the $\cal Q$-cohomology, which form ${1\over 16}$-BPS defect networks. By performing the $\cal Q$-localization of the ${\cal N}=4$ SYM on the four-dimensional hemisphere, we discover a novel defect-Yang-Mills (dYM) theory on a submanifold given by the two-dimensional hemisphere and described by (constrained) two-dimensional Yang-Mills coupled to topological quantum mechanics on the boundary circle. This also generalizes to interface defects in ${\cal N}=4$ SYM by the folding trick. We provide explicit dictionary between defect observables in the SYM and those in the dYM, which enables extraction of general ${1\over 16}$-BPS defect network observables of the SYM from two-dimensional gauge theory and matrix model techniques. Applied to the D5 brane interface in the $SU(N)$ SYM, we explicitly determine a set of defect correlation functions in the large $N$ limit and obtain precise matching with strong coupling results from IIB supergravity on $AdS_5\times S^5$.